Add links to detailed help pages for errors and warning where applicable

[baronfel]

Once https://github.com/microsoft/vscode/issues/11847 lands, we should be able to update the diagnostics api to include the new target property and easily link up things like FSharpLint error codes and F# Compiler error messages to their target help pages.

[Krzysztof-Cieslak]

This is now tracked in LSP repo here - https://github.com/microsoft/language-server-protocol/issues/926