add handler function for julia/help request

[ZacLN]

handler for julia-vscode/julia-vscode#818

[aviatesk]

The pain point of this approach is that we can't get documentation for user code.
What do you think about using our REPL session for this ? I have a working implementation that works for any user package code as well as usual script.

[davidanthoff]

I do think we should be able to get the documentation for user code in the LS process because we have a live parsed representation of every file in the workspace here, and that should include any user docs.

But, I don't know how :) Maybe looking at https://github.com/julia-vscode/LanguageServer.jl/blob/master/src/requests/hover.jl might clarify it? Otherwise, @ZacLN would know :)

[ZacLN]

You can pull explicit documentation from the CST easily as in David's link above