fix: dedupe extensions array in JS-converted solid-v2 vite config

[ryansolid]

Summary

• Found during Failed to load url ~/entry-server.tsx (trying to find .tsx instead of .jsx) #73's verification: scaffolding the Solid 2.0 basic template with --js produced extensions: ['.jsx', '.jsx'] in the converted vite.config.js. The TS template lists extensions: ['.jsx', '.tsx'], and the blanket .tsx → .jsx retarget regex added in feat: Solid 2.0 template support #82 rewrites the second entry into a duplicate of the first.
• Extracted the retarget into a retargetTSFilenames helper that dedupes the extensions array after the rewrite — the converted config now reads extensions: ['.jsx'].
• Also retargets .ts/.tsx filename mentions in the config's comments and the template README (e.g. src/App.tsx → src/App.jsx, `vite.config.ts` → `vite.config.js`), since every such file is renamed by the conversion. .d.ts mentions are excluded — those files are deleted, not transpiled. .tsx mentions inside transpiled src/ sources (JSX text/comments in the route files) are intentionally left as is; rewriting sucrase output in the shared conversion path is out of scope for this fix.
• No changeset: like fix: use @solidjs/web as jsxImportSource in solid-v2 jsconfig #83, this only touches unreleased code already covered by the pending solid-v2-templates changeset for 0.9.0.

Test plan

• Root pnpm test — 18/18 passing (16 existing + 2 new unit tests for the retarget against the solid-v2-basic fixture)
• Re-scaffolded solid-v2 basic with JS conversion from the local build: config is extensions: ['.jsx'], no stale .tsx in config/README
• npm run dev on the converted app: / and /users serve 200, and /src/routes/index.jsx?pick=default&pick=route compiles through the Solid transform (the extensions option's purpose)

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