Implement a setting for only using balloons for LSP/tsserver hovering

[w0rp]

The balloonexpr function ALE offers has the dual purpose of showing diagnostic messages, which long predate the existence of Language Server Protocol, and also for showing information about the symbol at the cursor via tsserver and LSP servers. I want to add a setting to make mouse hovering only show the "hover" information for symbols, and never the errors. This should be a simple change to make, by supporting a string for g:ale_set_balloons for this mode. See #2697 for a related change that would make sense to implement at the same time.

I had considered having a setting for reversing the priority of message for a while, but the asynchronous nature of how hovering works with the client-server communication would make the user experience less than good. Therefore, I'm content to just add a setting for turning the balloon diagnostics off.

[w0rp]

You can now do something like this to use balloons only for hover information.

let g:ale_set_balloons = has('gui_running') ? 'hover' : 0