fix: non-QWERTY keyboard layouts need the UI event's key property ins…

…tead of the code property # Description At least for Xorg@Linux, the keydown event provides two relevant properties: - code - key [ Cf. https://w3c.github.io/uievents/tools/key-event-viewer.html ] _code_ is based on „standard” QWERTY keyboard layout, whereas _key_ reflects/respects the user's keyboard layout. Prominent example for German QWERTZ layout: pressing z (located on the key for y on a QWERTY layout) leads to this situation: pressed | key | code ------------------------- Z | z | KeyY Y | y | KeyZ This behavior is obviously worse for Dvořak or Neo layouts and the like. Unfortunately, just changing to the _key_ property is not feasible because _key_ does not provide the useful keycodes, e. g. `ShiftRight`. On the other hand, just adding a matching line for the previous mapping of event _codes_ lead to conflicts with special character keys remapped to alphanumeric letters, e. g. in Neo layout the letter ›y‹ is produced by the key producing a single quote on QWERTY keyboards, which would have led to lack of C-y and M-y for emacs mode keybindings. The patch / pull request goes a middle ground by special handling of alphanumeric characters without modifying the handling of special characters. It's highly probable that it needs further special treatment for characters like / (slash) or # (hash). It's only tested on QWERTY, QWERTZ, and Neo layouts. Further testing is necessary, esp. for various country-specific layouts (French, Italian, Greek, …), but the patch will allow international / non-QWERTY users to start using Nyxt4. Fixes #3677 # Checklist: - [x] Git branch state is mergable. - [ ] Changelog is up to date (via a separate commit). - [ ] New dependencies are accounted for. - [ ] Documentation is up to date. - [ ] Compilation and tests (`(asdf:test-system :nyxt/<renderer>)`) - No new compilation warnings. - Tests are sufficient for Linux with Xorg. Not Wayland on Linux, not MacOS.