Add support for diagnostics display funcref for more flexible integration

[martskins]

This PR provides a config option for a function name to be used as a callback for the publish diagnostics call. What this does is enable us to integrate with external plugins that are used to display diagnostics, such as dense-analysis/ale. The documentation includes an example of how to set that up.

Note that the virtual texts will still be displayed if configured.

Fixes #800 and #852.

[martskins]

This one is a little more involved or disruptive so I'll leave it to you to decide @autozimu, but I think it's a good way to integrate with ale. I think in our case, given that the server publishes the diagnostics when it pleases, we don't need to answer the ALEWantResults autocmd, so this should be enough.

[martskins]

Maybe we should reuse the diagnostics_list config instead? And allow the values to be either Quickfix, Location, Disabled or a Funcref ? I think that makes more sense.

[martskins]

Pushed another commit to remove that config and replace it for support for funcref on LanguageClient_diagnosticsList. Made some changes to how selectionUI works in those terms as well, for the sake of uniformity.