fix(linux): respect user keyboard layout via xkbcommon

[tharropoulos]

• add xkbcommon dependency behind both wayland and x11 features
• replace the hard-coded US keymap in keycode_to_key with xkb::State::key_get_utf32 so character keys honour the active layout
• consume Wayland Keymap/Modifiers events to build and update an xkb::State from the compositor's keymap
• enable the xkb feature on x11rb and drive an xkb::State from the X server's keymap and XkbStateNotify updates
• drop the manual shift_held tracking and shifted-character lookup tables in both backends
• fix Alternative keyboard layout compatibility #38

[ploMP4]

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