Add support for F1 help link navigation

[dibarbet]

VS has a feature to navigate to msdn documentation by hitting F1 when the cursor is on a particular keyword or core library reference. We should consider porting this feature to work in VSCode.

We will likely need a different shortcut as F1 opens the command palette.

[JoeRobich]

We could do this ourselves, but wonder if it should be a platform feature which requests a help URL from the language server.

[dibarbet]

We could do this ourselves, but wonder if it should be a platform feature which requests a help URL from the language server.

We could also prototype ourselves and once we have something working propose it for the LSP spec