Lib mode should export types

Clear and concise description of the problem

Previous issues: #2049, #2989

To provide a better experience for our users, lib mode should automatically generate TypeScript definitions, and not require additional setup.

Suggested solution

Alternative

Workarounds:

• @keelii https://github.com/vitejs/vite/issues/2989#issuecomment-820914375

  {
    "build:types": "tsc -m amd -t ESNext -d --isolatedModules false  --moduleResolution node --esModuleInterop --jsx react --outFile dist/index.js src/main.ts && rm -rf dist/index.js "
  }
  

• @sanjade https://github.com/vitejs/vite/issues/2989#issuecomment-826064324

  {
    "build": "tsup index.ts --format esm,cjs,iife --dts --minify"
  }
  

Additional context

[qmhc]

I made a plugin vite-plugin-dts to generate d.ts for lib.

[aleclarson]

due to we would need to rely/depend on typescript, and we don’t want that dependency.

A direct dependency isn’t required, since we can use resolve to find typescript from the project root. Regardless, we still think this use case should be handled in a userland plugin, if anything. We’d like to avoid the maintenance burden and subtle complexities introduced by declaration bundling. Someone in the community will hopefully create a plugin if the mentioned workarounds are insufficient.