This commit adds some workbench color keys for text related elements
that were not supported by Nord yet.
The commit in the VS Code repository (1) added multiple new theme key
in through microsoft/vscode#26298 that was
merged over 2 years ago and released for the first time in stable
version 1.13.0 (2) but were never mentioned in the release notes.

The following theme keys have now been added:

- `textBlockQuote.background` - Background color for block quotes in
  text.
- `textBlockQuote.border` - Right-side border color for block quotes in
  text.
- `textLink.activeForeground` - Foreground color for links in text when
  clicked on and on mouse hover.
- `textLink.foreground` - Foreground color for links in text.
- `textPreformat.foreground` - Foreground color for preformatted text
  segments like inline code.
- `textSeparator.foreground` - Color for text separators.

References:
  (1) https://github.com/microsoft/vscode/blame/752acd504206f00ffe4ed101c2496540d0197f56/src/vs/platform/theme/common/colorRegistry.ts#L164-L170
  (2) https://code.visualstudio.com/updates/v1_13

GH-154