`sv add` creates a `vite.config.ts` even if `vite.config.js` exists

It looks like `sv add` assumes it should write to a `vite.config.ts` file if it detects that it's in a TypeScript project, even if `vite.config.js` already exists. To reproduce:

```bash
npx sv create vite-config-repro --template=minimal --types=ts --no-add-ons --install=pnpm
cd vite-config-repro
mv vite.config.ts vite.config.js
npx sv add devtools-json --install=pnpm
ls vite.config.*
```