Let the user choose handeling of non ascii characters for shortcuts

[Gerd-Augsburg]

Hi,
i start with my perspective on this topic:
I use an ergonomic german keyboard layout (Neo2). So it is mostly ascii but with our german "äüöß".
Vim motion keys are assigned by position not by value. On my layout the keys are rearanged and for example the motion keys are not very convenient to use any more. If you reassign those you have conflict with other key, than you reassign those leads to other conflicts and so on.
So i started to create the shortcuts from scratch. A very nice option Zed has.
This worked very well and Zed had the best handling of all the keys, higher layered keys, non ascii etc. all just worked as expected.

Than came PR #34053
Some problems got fixed before they hit me, but you missed at least one:
Please add "Insert" to the special keys in keysym translation in crates/gpui/src/platform/linux/platform.rs impl Keystroke::from_xkb like the arrow keys.
Additionally now i cant use "üöäß" for shortcuts any more but they have a nice placement.
I can use the keys but they got different values, values from other keys and i used those for other things.

My suggestion is: let the user choose how to handle their special keyboard and i suggest 3 "key_mapping" options:
"all": Ignore the local keyboard and assume it is a "en" one. Good for ergonomic keyboards and vim motion.
"non_ascii" (default): the current state, make predefined layouts work with minimized confusion on non ascii chars.
"none": The handling as it was before. Use the keys as they are and let the user config the shortcuts that don't work or let them start from scratch.