Possible fix for issue #4642: use ModeChanged events instead of InsertLeave emulation

[zlogic]

ALE is my favourite way of using LSPs and linters, the only thing that might need improvement is #4642. Any time I press Esc and start typing or moving around the document (muscle memory :) ), Vim beeps and Tried switching to NeoVim and Helix, only to return back to Vim+ALE :)

I've done some research/testing during this weekend, and it looks like Vim 9 has some issues (a timeout reset?) if a timer is started from an InsertEnterEvent.
InsertEnter is called before switching into Insert mode, so mode() might potentially return "normal" mode if the timer is triggered too early.
I was able improve the situation by specifying a shorter timeoutlen, but that doesn't seem like an ideal solution. When Esc or\<C-[\> is pressed, Vim waits some time to see if it's an escape code for something else (like an Arrow Up key).

ModeChanged looks like a more reliable way to detect an "exit insert mode" event and is a lot simpler (doesn't need a timer). Also, it can detect some other transitions like \<C-o\> in insert mode.
The ModeChanged event is available in:

• Vim 8.2.3430
• NeoVim 0.7.0

so the current tests are covering both InsertMode emulation and the new ModeChanged events.

With this PoC, the situation has improved significantly on my machine - no more beeping, and exiting Insert mode is instantaneous.

[zlogic]

Or it might make sense to just make the InsertLeave emulation optional (configurable), perhaps some developers might use Esc to run the linter and other event handlers, but use \<C-c\> to temporarily exit Insert mode without any possible side-effects/callbacks.