For better ESM compatibility, this adds a file extension for nested
package files.
This also removes the nested special handling of
`vscode-languageserver-types` and `vscode-languageserver-document`
imports. This is unnecessary, because they use the `"module"` field in
`package.json`.
- ESLint config `remcohaszing/typechecking` is extended.
- Various previously disabled ESLint rules have now been enabled.
- Various `any` types have been fixed.
- Removed useless type check of diagnostic code.
- The diagnostics adapter listener has been turned into an actual map.
TypeScript is better at inferring types when using simple functions and
objects instead of classes. As a result many type annotations have now
been removed without impacting type safety.
Some other benefits are this can be minified better and private fields
are now truly private variables.
