Allow LSP packages to customize format_completion

[predragnikolic]

Currently the format_completion tries to have a one size fits all way to display LSP CompletionItem information.
That proven to not be ideal for some LSP servers.

While I am ok with the way that format_completion currently works,
it makes sense to expose an API to LSP-packages
to allow changing what will be used for the Sublime Text CompletionItem.trigger, CompletionItem.annotation, and CompletionItem.details.

Note to avoid performance issues, because format_completion can be called often,
the api must take that into account.

related:
#1924
#569
#2293 - we might allow LSP-* packages to alter the label
#1833 (comment) - see if we can avoid this issue as well