Reduce number of d.ts files and types published in packages

Problem

The published npm package for vscode-languageclient currently includes a d.ts file for every .js file under lib:

This is problematic for two reasons:

• It bloats the package size. All the d.ts files in vscode-languageclient take up 300KB or so

• We are likely publishing interfaces and types that are not meant to be exposed externally. For example, there are files like utils/async.d.ts.

Proposed fix

To fix this, I suggest adopting a tool such as api extractor. This lets you bundle all the types for a project into a single d.ts file. You can also control which types are exposed and which ones stay internal

I adopted this for the markdown language service: https://github.com/microsoft/vscode-markdown-languageservice/blob/main/api-extractor.json

This helped me make sure I only exposed a well defined API to library consumers

Start by inspecting the declarations generated for lib, including the example utils/async.d.ts, and compare the proposed approach with the referenced api-extractor.json configuration. Done means the published packages expose only the intended types, consolidate declarations, and reduce the current package bloat.