fix(highlight): highlight Svelte components (#3487)

* fix(highlight): highlight Svelte components

`.svelte` had no entry in the extension table, so `highlightCode` fell through
to one unstyled token per line. That is the same table the file pane, the diff
tokens, markdown fences and the web editor all resolve through, so a component
rendered as plain text everywhere in the app.

Register `@replit/codemirror-lang-svelte`. Its `svelteLanguage` already wires
the mixed parsing for the `<script>` and `<style>` blocks, so the script gets
JS/TS roles, the style block gets CSS roles, and the template gets tag,
directive and mustache roles. It comes from the same publisher as the
`@replit/codemirror-lang-csharp` entry already in this package, and only the
grammar itself is declared: its CodeMirror peers resolve the same way the C#
grammar's peers already do.

* test(server): cover Svelte diff highlighting

The review surface in the issue is the diff, and the daemon is what tokenizes
it (`diff-highlighter.ts` calls `highlightCode`), so the regression belongs
here too: script, template block and markup roles on a `.svelte` hunk.

packages/highlight/package.json

@@ -47,6 +47,7 @@
     "@lezer/yaml": "^1.0.4",
     "@replit/codemirror-lang-csharp": "^6.2.0",
     "@replit/codemirror-lang-nix": "^6.0.1",
+    "@replit/codemirror-lang-svelte": "^6.0.0",
     "lezer-elixir": "^1.1.2"
   },
   "devDependencies": {