Add types field in package.json exports

[AkaraChen]

Is your feature request related to a problem? Please describe.
Could not find a declaration file for module 'y-codemirror.next'.
There are types at /dist/src/index.d.ts', but this result could not be resolved when respecting package.json "exports". The 'y-codemirror.next' library may need to update its package.json or typings.

Describe the solution you'd like
Add types field

Describe alternatives you've considered

Additional context

[lorefnon]

@dmonad Please merge this change. Without this change this library can not be used in a project that uses moduleResolution: bundler (recommended now for all ts projects that use a bundler) in tsconfig.