Package code with webpack

[evidolob]

What does this PR do?

Add package yaml-ls code with webpack. New bundled script is in./out/server/language_server.js.
Bundle contains all yaml-ls dependencies except prettier. As it quite huge(~ 10Mb scripts) and including it in webpack bundle breaks debugging.

Note: To keep backward compatibility, I left compilation with pure ts, so old ./out/server/src/server.js should work.

What issues does this PR fix or reference?

redhat-developer/vscode-yaml#521

Is it tested? How?

[coveralls]

Coverage increased (+0.1%) to 78.474% when pulling 83b27df on evidolob:use-webpack into 40b6fe4 on redhat-developer:main.