fix(auto-insert): restore content of context instead of undo

[xzbdmw]

fix #942
fix #810

Described in #942 (comment), this pr removes the preview_undo field and make the auto_insert stateless.

[xzbdmw]

Blink has the same issue with nvim-cmp when used with mini.snippets, but should not related to this pr, I'd open a new one.

[xzbdmw]

I feel this function is unnecessary (keep it a blank body also fix these two issues), we already handles text_edits smartly, it says TODO: doesnt work when the item contains characters not included in the context regex, but I have not find a condition this guess fails.

This pr sort of emulate the restore functionality by deleting chars until list.context.bounds.start_col

[Saghen]

This is a cool idea, I'd love for this to be stateless! Unfortunately though, it's not enough to use the start_col of the bounds since the edit range of the text edit may not equal those bounds. I believe the most correct solution is still to undo the text edit explicitly, there's a few bugs remaining in this approach though

[xzbdmw]

Yeah you are right, they are not always match, nvim-cmp has a function to compute the offset when cycling completion list with auto-insert based on TextEdit and the offset when request was sent (source_offset). So what it does is to delete from cursor col until this offset.