Refactor ad-hoc hooks to use the event system

[the-mikedavis]

This is meant to be a tracking issue for refactoring existing ad-hoc hooks to use the event system introduced in #8021.

• LSP completion (Add an event system #8021)
• LSP signature help (Add an event system #8021)
• LSP inlay hints
• Diff gutter
• LSP document change notifications
• Picker preview highlighting (Pickers "v2" #9647)
• DynamicPicker callback (Pickers "v2" #9647)
• File writes
• Auto formatting
• LSP completionItem/resolve (Use a hook for resolving completion items #9668)

[pascalkuthe]

we may also want to update mapping diagnostic change (and inlay hint mapping and so forth) to use the event system. Really most of the custom code in apply_impl could be moved there.