Fixes #515

Hello, and Happy Christmas 🎄

This PR iterates on #2630 with the given feedback of not requiring the use of the markdown filter to execute nested shortcodes. Additionally, unlike the previous PR, nested nth invocation increments follows a more intuitive pattern.

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After:

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Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
  • This PR replaces Allow nested shortcodes with markdown bodies #2630

Code changes

• The shortcode parser has been changed to stop at the appropriate {% end %}, instead of the first-encountered {% end %}. (Thanks to Allow recursively nested shortcodes #1475)
• A clonable shared ShortcodeInvocationCounter has been added
• The markdown::shortcode::parser::Shortcode struct now has inner: Vec<Shortcode>
• The markdown::shortcode::parser::parse_for_shortcodes function now also parses shortcode bodies
• Added a flatten method to markdown::shortcode::parser::Shortcode which renders the inner-shortcodes into the shortcode body
• The render method on markdown::shortcode::parser::Shortcode now calls flatten

• Are you doing the PR on the next branch?

If the change is a new feature or adding to/changing an existing one:

• Have you created/updated the relevant documentation page(s)?
  • As far as I'm aware, the documentation does not explicitly prohibit nested short-codes.

Let me know what you think!