fix(plugin-legacy): add types export

[ocavue]

Description

After updating @vitejs/plugin-legacy to v7.0.0, TypeScript is complaining about not being able to find type declarations:

This is because @vitejs/plugin-legacy v7.0.0 removed the "types" field in package.json. Noice that there isn't any information about dist/index.d.ts in v7.0.0 package.json.

@vitejs/plugin-legacy v7.0.0 package.json:

  "exports": "./dist/index.js",

@vitejs/plugin-legacy v6.1.1 package.json:

  "main": "./dist/index.cjs",
  "module": "./dist/index.mjs",
  "types": "./dist/index.d.ts",
  "exports": {
    ".": {
      "import": "./dist/index.mjs",
      "require": "./dist/index.cjs"
    }
  },

This PR fixes the issue by adding back "types": "./dist/index.d.ts" to package.json. This matches vite@7.0.0's package.json structure.

[sapphi-red]

TypeScript will use exports field and loads the corresponding dts if moduleResolution is set to node16+ or bundler. So this is only needed for moduleResolution: node/node12.
But I'm ok with adding this back.

[bluwy]

Wouldn't setting that moduleResolution be incorrect anyways since our engines field already specifies node 20/22? I don't think we have to support that usecase, and pushing to fixing on user end side should also fix other things.

[ocavue]

Well, since vite@7.0.0 still includes the types field in its package.json, I believe it's reasonable to add back types to @vitejs/plugin-legacy.

If the Vite team want to push the community to update/fix their tsconfig.json, it might be more effective to update all Vite packages simultaneously and provide a migration path in the release notes etc.