feat: add support to astro syntax

[felixicaza]

What does this change?

This PR adds a new grammar to support .astro file syntax based on the Astro Language Tools syntax definition, and reuses MicroLighter's existing HTML, CSS, SCSS, TypeScript and TSX grammars for embedded content.

Astro files combine a TypeScript frontmatter section with an HTML/JSX-like template, where expressions can be embedded directly in markup and <script> and <style> blocks can contain JavaScript or TypeScript, CSS or SCSS respectively.

This grammar includes:

• Astro frontmatter.
• <script> and <style> blocks.
• Interpolations.
• Embedded HTML/CSS/SCSS/TypeScript/TSX syntax.

Other reference:

• SYNTAX_SPEC.md

Playground preview:

Checklist

• npm test passes locally
• The size budget still passes (note any size impact below)
• Added/updated tests or the demo (docs/index.html) if behavior changed
• For a new grammar/theme, followed the steps in CONTRIBUTING.md

Size impact

Bundles
                               file       raw      gzip    brotli
  ---------------------------------  --------  --------  --------
           dist/microlighter.min.js  4.52 KiB  2.09 KiB  1.91 KiB
  dist/micro-lighter-element.min.js  8.81 KiB  3.59 KiB  3.17 KiB

Lazy-loaded chunks (fetched on demand)

                        file  raw (min)      gzip
  --------------------------  ---------  --------
           grammars/astro.js   1.61 KiB  0.49 KiB

Average chunk size

  category  files  raw (min)      gzip
  --------  -----  ---------  --------
  grammars     38   1.69 KiB  0.73 KiB

Budget: dist/microlighter.min.js gzip 2.09 KiB / 2.10 KiB (ok)

[davatron5000]

@copilot resolve the merge conflicts in this pull request

Lol. this did nothing.

[davatron5000]

Thanks @felixicaza! Got it in. Will cut a release soon'ish.