feat: add Svelte language support (tree-sitter + svelte-language-server)

[jafreck]

Summary

Add Svelte support via tree-sitter + Svelte Language Server, leveraging existing TypeScript infrastructure.

Tooling

Component	Tool	Notes
Tree-sitter grammar	tree-sitter-svelte	Community maintained
LSP server	svelte-language-server	Official Svelte team, active
SCIP indexer	None	N/A

What to implement

Phase 1: Script block extraction

• Add tree-sitter-svelte as an npm dependency
• Create src/indexer/extractors/svelte.ts:
  • Parse the Svelte component structure to locate <script> and <script context="module"> blocks
  • Delegate <script> content to the existing TypeScript/JavaScript extractor for symbol and call-ref extraction
  • Extract export let propName as prop/interface symbols
  • Extract $: reactive declarations as computed symbols
  • Extract import statements from <script> as file imports
• Register svelte in SourceIndexStage EXTRACTORS map and ParserPool LANG_PACKAGES
• Add svelteserver to src/indexer/lsp/registry.ts: { command: 'svelteserver', args: ['--stdio'] }
• Add file extension .svelte to walker
• Add test fixtures in tests/fixtures/svelte/
• Add extractor tests

Phase 2: Template component references (follow-up)

• Extract <ComponentName /> usage in the template markup as call refs
• Extract on:event handler bindings as refs
• Extract bind:prop and slot references

Language notes

• Svelte components are single files with <script>, markup, and <style> sections
• <script context="module"> runs once per module (not per instance) — different scoping
• export let in <script> declares component props
• $: reactive statements re-run when dependencies change — these should be tracked
• Svelte 5 introduces $state(), $derived(), $effect() runes — new patterns for the extractor
• The Svelte language server (svelteserver) is an npm package: npm install -g svelte-language-server

Context

Svelte is a top-5 web framework with growing adoption. Like Vue, Svelte components contain TypeScript/JavaScript logic that the existing TS extractor handles. The main value-add is SFC structure recognition and component reference tracking.