chore: use InsertLeave instead of TextChangedI

[argshook]

hey there!

thank you for this superb plugin, it's extremely useful!

i had a small issue with undoing changes done in insert mode and took
few minutes to fix it.

Not sure if it's the best way to fix it, and i haven't thoroughly tested
all features (there're a lot!).

gp.nvim has autosave feature which is triggered by few events, one of which is TextChangedI.

when typing text in insert mode, any change done there saves the file. This grows the :undolist too much, because any change in insert mode is saved and treated as a separate change.

Normally to undo the last change, you would press u once in normal mode. But with autosave triggered by TextChangedI, you have to press u multiple times, for each change.

using InsertLeave instead of TextChangedI avoids such problem.

For comparison, consider scenario:

1. open neovim
2. run :GpChatNew
3. go to insert mode and type 123
4. leave insert mode
5. run :undolist

on my setup it shows:

number changes  when               saved
     4       4  2 seconds ago

doing the equivalent when InsertLeave is used instead of TextChangedI:

number changes  when               saved
     1       1  2 seconds ago        1

this is much better, because now you can undo the last change with single u press!

[Robitx]

@argshook hey, thanks for the PR! InsertLeave in the first instance is a nice improvement, the second use of TextChangedI actually has a purpose (so that ChatPicker can dynamically grep and update listing of old chats).