Start to split out initialization and registration

[ConradIrwin]

Still TODO:

• Factor out start_language_server so we can call it on register (instead of on detect language)
• Only call register in singleton editors (or when editing/go-to-definition etc. in a multibuffer?)
• Refcount on register so we can unregister when no buffer remain
• (maybe) Stop language servers that are no longer needed after some time

Release Notes:

• Fixed language servers starting when doing project search
• Fixed high CPU usage when ignoring warnings in the diagnostics view