Pack extension

[DoctorKrolic]

• First commit: moved language server into root extension directory. I wasn't a fan of mixing js output with language server build from the very beginning, this is a good opportunity to change that
• Second commit: actually applied webpack for extension build. Now resulting vsix size dropped from 16MB to just 10MB
• Thrid commit: simplified source directory structure, so the actual client code is directly in src folder, not src/extension (I guess, this was a legacy from the time when language server and extension client code were in the same repo)

Verified, that debugging of extension client is working. Verified, that vsce package produces a valid vsix, so if extension is installed from that vsix it is fully functional.

[DoctorKrolic]

@tintoy I encourage you not only to review the code but to download this branch locally and make same verifications as I did (try to launch extension in debug mode and ensure breakpoints are hit; build vsix, install it locally and ensure extension works) to double-check that everything is working as expected

[DoctorKrolic]

I'm getting a webpack warning whn building extension. Filled https://github.com/microsoft/vscode-languageserver-node/issues/1355. Gonna hold this until we get a confirmation, that this is safe and we aren't gonna break anything