Allow providers to disable themselves

[stefanboca]

I'm proposing to add a function

--- @field enabled fun(self: blink.cmp.Source, context: blink.cmp.SignatureHelpContext): bool

to blink.cmp.Source, that will allow sources to disable themselves.

I know there is already the function should_show_items on sources, but from what I understand this only hides items after a source generates completions. I also see that there is an enabled field in blink.cmp.SourceProviderConfig, but that seems to be only configurable by the user and not the source itself.

However, there are some completion providers that only make sense on, for instance, a specific file type, so having them run on all file types would be wasteful for performance.

This looks pretty simple to implement - adding the function to the source, then updating sources.get_enabled_providers to check the result of the function. If you'd like, and you agree with the change, then I can quickly submit a PR with the changes.

[stefanboca]

Thanks!