fix: include types in package.json exports

[joelmukuthu]

This fixes this typescript error, when one tries to import windiCSS from 'vite-plugin-windicss';:

Could not find a declaration file for module 'vite-plugin-windicss'. 
  '/project/node_modules/vite-plugin-windicss/dist/index.mjs' implicitly has an 'any' type.
  There are types at '/project/node_modules/vite-plugin-windicss/dist/index.d.ts', but this 
  result could not be resolved when respecting package.json "exports". 
  The 'vite-plugin-windicss' library may need to update its package.json or typings.ts(7016)

This happens with vite-plugin-windicss v1.9.0, typescript v5.1.6 and the moduleResolution tsconfig compiler-option set to bundler, which per my understanding makes typescript import stuff only from what's defined in a module's exports.