Implement language server protocol client

[huguesv]

https://docs.microsoft.com/en-us/visualstudio/extensibility/adding-an-lsp-extension?view=vs-2017

If this works out it could greatly reduce the amount of IntelliSense and other editor code we have to maintain.

Some of our special needs are not necessarily well tested / supported, so there are risks:

• Designed to work for open folder, not projects, so we need to do custom initialization for projects
• Colorization supported by TextMate, should be able to use our own colorizer
• Interactive window completions require fetching from both language server and live running session

This will require porting our custom functionality that isn't supplied by language client. We'll need to move our own Analysis protocol as language server protocol extension and access it via the LSP client connection.

[huguesv]

Closing in favor of #5210