Functions support

[yaroslavyaroslav]

ChatGPT functions call support #17 is implemented in messages-refactoring branch.

Currently it supports just three hardcoded functions get_region_for_text, replace_text_for_region and read_region_content.

Worth to say, that as for my internal testing with gpt-4o it's not that useful as it suppose to be to be and the reason of that is the gpt itself. It's quite dumb about getting the point where to put content within the region (to deduct the right offset regarding the beginning of the region), so I believe there's some additional prompt hacking required here. But still this is quite something.

Currently it's in a state where golden path only is implemented, thus it's not ready to be merged and released. But if some of you eager to try this, feel free to manually clone the repo and switching the branch. As usual for this repo, every commit in feature branch is a non broken one.