feat: keycode resolution, keycode triggers

[tyrypyrking]

Solves the issues with multiple layouts and exotic xkb keymaps behaving undesirably. By allowing for binds to physical keys and/or resolving all keybinds against default xkb layout. This allows niri to match behavior of majority of applications in regards with keysym resolution and mapping.

Related discussions: #3255 (solves), #2134 (useful proposal, but stale), #1919 (solves),
Related issues: #283 (solves)
Similar PR: #1920 (draft)

Note on variants: Pull request contains two variants for the same functionality provided in two commits. Cleaner (var2) and "Safer" (var1). Var1 is not much safer because avoiding xkb mutex lock and unwrap doesn't yeild any significant performance, nor will it remove other unwraps used by the libraries handling xkb connection.

[Jappie3]

works with my corne kb that uses a layout that doesn't need to be interpreted as dvorak (unlike my laptop keyboard), thanks!

combined with #3061 to set different layouts for both:

input  {
  bind-to-keysyms
  keyboard  {
    numlock
    xkb  {
      layout "us"
      variant "dvorak"
    }
  }
  keyboard "ZMK Project Corne Keyboard"  {
    xkb  {
      layout "us"
      variant ""
    }
  }
}