feat!: implement auto-insert option

[noomly]

Closes #34

Still left to do:

• Figure out a less dirty way of identifying whether a character update comes from selecting rather than accepting an item (see trigger.triggered_by). I'm open to suggestions on this one.
• Enforce preselect = false when auto_insert = true. We could turn preselect off when auto_insert is on and print a message on setup warning the user about the incompatibility, wdyt?
• Maybe call accept when implicitly accepting a completion item by selecting it and pressing a non-completable character (like ( for ex) as mentioned here Allow auto-inserting completion items #34 (comment).
• Handle snippets

[tkapous]

Maybe there should be only one option for selection behaviour, e.g select: 'auto_insert' | 'preselect' | nil?

[noomly]

Maybe there should be only one option for selection behaviour, e.g select: 'auto_insert' | 'preselect' | nil?

That's a much better idea than having two separate booleans yes! Will implement that tomorrow.

[noomly]

I think the PR is ready to be reviewed @Saghen. I'm still not sure about the first checklist point, and I also added another comment.

I tested as much edge cases as I could think of withvtsls, luals and rust_analyzer but it's definitely possible that I missed some which could end up botching up the insertion when auto-insert is on. I'll be watching the issues for related bugs once this PR is merged.

[Saghen]

I've added a wrapper on the trigger module so we can edit the buffer without triggering the sources/fuzzy 19e1312. Wicked job on this PR, thank you!