Problem:  exception handling can be improved
Solution: add v:stacktrace and getstacktrace()

closes: vim/vim#16360

https://github.com/vim/vim/commit/663d18d6102f40d14e36096ec590445e61026ed6

Co-authored-by: ichizok <gclient.gaap@gmail.com>
Co-authored-by: Naruhiko Nishino <naru123456789@gmail.com>

- classes for v:event and v:completed_item
- add remaining unknown types

Problem:  Missing information in CompleteDone event
Solution: add complete_word and complete_type to v:event dict
          (glepnir)

closes: vim/vim#16153

https://github.com/vim/vim/commit/1c5a120a701fcf558617c4e70b5a447778f0e51d

Co-authored-by: Ernie Rael <errael@raelity.com>
Co-authored-by: Famiu Haque <famiuhaque@proton.me>
Co-authored-by: Jade <spacey-sooty@proton.me>
Co-authored-by: glepnir <glephunter@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Problem:  'findexpr' can't be used for lambads
          (Justin Keyes)
Solution: Replace the findexpr option with the findfunc option
          (Yegappan Lakshmanan)

related: vim/vim#15905
closes: vim/vim#15976

https://github.com/vim/vim/commit/a13f3a4f5de9c150f70298850e34747838904995

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

Problem:  'findexpr' completion doesn't set v:fname to cmdline argument.
Solution: Set v:fname to the cmdline argument as-is (zeertzjq).

closes: vim/vim#15934

https://github.com/vim/vim/commit/20e045f78148c0ef0143c33ffe686fee72d29376

Problem:  cannot easily adjust the |:find| command
Solution: Add support for the 'findexpr' option (Yegappan Lakshmanan)

closes: vim/vim#15901
closes: vim/vim#15905

https://github.com/vim/vim/commit/aeb1c97db5b9de4f4903e7f288f2aa5ad6c49440

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

Problem: `CompleteDone` currently does not specify the reason for why completion was done, which is problematic for completion plugins as they cannot know whether the event was triggered due to the completion being canceled, accepted, or for some other reason.

Solution: Add a `reason` key to `v:event`, which is set by `CompleteDone` to indicate why completion ended.

Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>
Co-authored-by: Famiu Haque <famiuhaque@proton.me>
Co-authored-by: Gregory Anders <greg@gpanders.com>
Co-authored-by: Guilherme Soares <guilhermesoares1970@gmail.com>
Co-authored-by: Jannik Buhr <jannik.m.buhr@gmail.com>
Co-authored-by: thomaswuhoileong <72001875+thomaswuhoileong@users.noreply.github.com>
Co-authored-by: tom-anders <13141438+tom-anders@users.noreply.github.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>