feat: add CTRL+[ binding as <Esc> alias

[liljaylj]

in Vim/Neovim there is alias for <Esc> mapping -- <C-[>. it is useful if your <Esc> key is hard to hit on your keyboard.

for reference:

• https://vimhelp.org/insert.txt.html#i_CTRL-%5B
• https://neovim.io/doc/user/insert.html#i_CTRL-%5B

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[akinomyoga]

1. on which keyboard is Esc hard to hit - just asking out of curiosity because I have only seen the Esc key in the top left corner: us-en, german, korean, spanish, ...

I actually always use C-[ to send ESC because I have difficulty in pressing ESC. Only when I use the keyboard of a laptop machine, without breaking the home position, I can reach ESC with my little finger. On all the keyboards of the full size, it's impossible for me to reach the ESC key.

2. does this shortcut work in terminal multiplexers?

This PR is only relevant when an advanced keyboard protocol is enabled. If the advanced keyboard protocol is enabled, it should work better in terminal multiplexers because the protocol is free from the heuristic and obscure distinction of Meta modifier vs ESC, which the terminal multiplexer needs to process. (But, I'm not sure crossterm supports enabling the advanced keyboard protocol of the outside terminal when terminal multiplexers are involved.)

[akinomyoga]

For the exactly same reasons as #1720, I agree to have this binding.

[akinomyoga]

ESC and C-[ are equivalent in the traditional terminal protocol, and we should assign the same functionality to both even in the advanced keyboard protocol unless we would drop the support for the traditional terminal protocols.

Given C-c and C-g already exit the Atuin session, another option would be to remove the keybinding to ESC, but that doesn't seem useful to me.

[ellie]

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