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/src/parsers.ts

@@ -16,6 +16,7 @@ import { parser as xmlParser } from "@lezer/xml";
 import { parser as yamlParser } from "@lezer/yaml";
 import { csharpLanguage } from "@replit/codemirror-lang-csharp";
 import { nixLanguage } from "@replit/codemirror-lang-nix";
+import { svelteLanguage } from "@replit/codemirror-lang-svelte";
 import { parser as elixirParser } from "lezer-elixir";
 import type { Parser } from "@lezer/common";
 
@@ -49,6 +50,8 @@ const languagesByExtension: Record<string, Language> = {
   // HTML
   html: language(htmlParser),
   htm: language(htmlParser),
+  // Svelte
+  svelte: svelteLanguage,
   // XML
   xml: language(xmlParser),
   // Java