This commit...

1. fixes `none` constant highlighting in `perspective()` functions
2. removes `<percentage>` from `matrix()` and `matrix3d()`
3. removes `<numbers>` from `translateX()` and `translateY()`
4. removes `<comma>` from `scaleX()`, ... .

5. adds and sorts transform related test cases.

Note: This commit intents to fix behavior/highlighting with existing
      accuracy/complexity/granularity.

      The decision to simplify those contexts by more lazy highlighting
      may/should be part of a future change.

This commit targets better extensibility of CSS by...

1. adding a `stylesheet` context which is the only one being included
   in `main`, so any extending syntax can easily insert e.g. frontmatter
   or shebang like prefixes.

2. adding named contexts for selector elements etc., so inheriting
   syntaxes can override or extend them more individually.

This commit does not have any effect on how CSS is highlighted.

This commit normalizes property name behavior by directly checking for
property values after a name. This has already be the case for `font`,
`font-family` and other special treated properties. It is now done for
all of them.

This change is not directly needed by CSS, but helps 3rd-party syntaxes
such as PostCSS, TailwindCSS, LESS, SCSS to implement nested rule sets.

This commit does not have any effect on how CSS is highlighted.

This commit enables interpolation of various identifiers
(e.g.:selectors, property names and constants) by not consuming
sequences of characters, but pushing dedicated identifier contexts onto
stack.

Notes:
- This commit does not have any effect on
  a) how CSS is highlighted
  b) parsing performance (interestingly)
- ASP, ERB, HAML, JSP and PHP are adjusted to benefit from new
  interpolation support and to illustrate how it works.
- Bash, HTML, YAML and maybe some other syntaxes follow that strategy
  already.

This commit adds named contexts for certain context-end patterns to

1. reduce some pattern duplication.
2. give inheriting syntaxes a chance to modify context bailouts more
   individually.

This commit does not have any effect on how CSS is highlighted.

The goal is to enable syntaxes to override/extend selector patterns
without overriding `selectors` or `selector-end` contexts directly.

This commit does not have any effect on how CSS is highlighted.

This commit fixes highlighting issues with property names beginning
with escaped characters by...

1. removing `property-identifier-content` context in favor of organizing
   the 6 remaining property name patterns directly in `rule-list-body`
   context.
2. simplifying property name patterns for performance reasons

This commit replaces `[:ascii:]` by `[\x{0}-\x{127}]` as it increases
overall syntax performance by about 13% in larger real world CSS files.

This commit...

1. adds named contexts for each builtin function
2. sorts contexts alphabetically within their logical sections
3. refactors function arguments list contexts by using multi-push in
   association with a common `function-arguments-list-begin` context.
4. adds missing case-insensitivity flags in some `gradient` function
   name patterns

These changes don't effect highlighting or parsing performance.

This commit renames `-list-content` contexts as `-body` is a common
naming scheme, which established for such contexts in other syntaxes.

These changes don't effect highlighting or parsing performance.

This syntax no longer has much in common with CSS3 package's syntax.

Another step towards less restrictive highlighting to reduce risk of
breaking syntax due to new/unknown functions.

This commit adds an organizational context, which most/all value related
expressions have in common. It can also be used as a common entry point
for extensions added by inheriting syntaxes.

The `builtin-values` and `common-operators` contexts are merged into the
new `values` context as they seem to provide little to no use anymore.

The `selector-content` context already includes comments and bailouts.
Hence no further prototype context is required.

1. replace terminator-pop, to avoid popping on parentheses.
2. prefer `function-arguments-prototype` over `value-prototype`
   in calc() arguments

That bundling may have unwanted effects on inheriting syntaxes.

Same fix as for angle-constants.

This commit enables inheriting syntaxes to interpolate layer names the
same way as all other identifiers.

Extend interpolation support to namespace prefixes of attribute names

Example (LESS):

  attrib: attr(n@{am}e|@{sp}ace|*|f@{o}o);

This commit applies context naming scheme to blocks, which proofed
itself valuable for extendable syntaxes in various places already.
 
All contexts follow the chosen naming scheme.

- ...-body
  - ...-end
  - ...-content

This commit applies context naming scheme to strings, which proofed
itself valuable for extendable syntaxes in various places already.
 
All contexts follow the chosen naming scheme.

- ...-body
  - ...-end
  - ...-content

All contexts follow the chosen naming scheme.

- ...-body
  - ...-end
  - ...-content

The additional named contexts increases syntax cache, but helps
inheriting syntaxes to extend selectors while maintaining bailouts and
comments at highest priority.

All contexts follow the chosen naming scheme.

- ...-body
  - ...-end
  - ...-content

All contexts follow the chosen naming scheme.

- ...-body
  - ...-end
  - ...-content

This commit restricts illegal line end matching to quoted strings and
provides named contexts for string-...-end.