Add Astro (`.astro`) lexer

[Xeonzilla]

Is there an existing issue for this?

• I have searched the existing issues

What is the missing lexer?

Astro components are commonly used in documentation sites, static sites, and blogs.

Astro files combine several syntactic regions:

• frontmatter/component script delimited by ---, usually TypeScript/JavaScript
• an HTML-like component template
• JavaScript/TypeScript/TSX-style expressions inside {...}
• Astro template directives such as client:*, server:*, class:list, set:html, set:text, is:*, transition:*
• embedded <script> and <style> blocks, including language/type-specific JS, TS, JSON, CSS, SCSS, Sass, Less, Stylus, etc.
• component tags such as <MyComponent /> in addition to normal HTML/custom elements

A basic lexer would already be useful if it recognized .astro, highlighted the frontmatter as TypeScript/JavaScript, the template as HTML-like markup, and {...} template expressions as TSX/TypeScript where possible.

Links to existing syntax definitions

Current official Astro language tools in the Astro monorepo:

• https://github.com/withastro/astro/blob/main/packages/language-tools/vscode/syntaxes/astro.tmLanguage.src.yaml
• https://github.com/withastro/astro/blob/main/packages/language-tools/vscode/syntaxes/astro.tmLanguage.json
• https://github.com/withastro/astro/blob/main/packages/language-tools/vscode/package.json
• https://github.com/withastro/astro/tree/main/packages/language-tools/vscode/test/grammar/fixtures

Astro syntax/documentation references:

• https://docs.astro.build/en/basics/astro-components/#the-component-template
• https://docs.astro.build/en/reference/astro-syntax/
• https://docs.astro.build/en/reference/directives-reference/

Additional grammar reference:

• https://github.com/virchau13/tree-sitter-astro