V2: Remove Node10 subpath fallbacks

[timostamm]

Active support for Node.js v10 ended nearly four years ago, security support ended nearly three years ago. I think we can safely remove the fallback for subpath exports in v2.

For context, see #802.

We still maintain the package.json field typesVersions so that TypeScript can see types with Node10 module resolution (it's still commonly used with the alias node). But there doesn't appear to be an obvious reason to maintain JS files for runtime purposes.

[timostamm]

We don't need to ship our tsconfig.json and src. The result:

$ npm pack -w packages/protoc-gen-es
npm notice 
npm notice 📦  @bufbuild/protoc-gen-es@2.0.0-alpha.1
npm notice === Tarball Contents === 
npm notice 5.0kB  README.md                           
npm notice 174B   bin/protoc-gen-es                   
npm notice 1.0kB  dist/cjs/package.json               
npm notice 14.5kB dist/cjs/src/protoc-gen-es-plugin.js
npm notice 5.5kB  dist/cjs/src/util.js                
npm notice 903B   package.json           
...