Add support for Multi_key in valid modifiers

[kevinclevenger]

I initially submitted this request to Niri and was redirected here,

To help work around some physical left hand issues I can set something like this in sway/swayfx

input type:keyboard {
  xkb_layout us
  xkb_variant altgr-intl
  xkb_options compose:rctrl
}

bindsym Multi_key+left exec wtype -P home -p home
bindsym Multi_key+right exec wtype -P end -p end
bindsym Multi_key+down exec wtype -P next -p next
bindsym Multi_key+up exec wtype -P prior -p prior

and get a 'right hand only' solution for home/end/pgup/pgdown on a laptop without having to use the fn modifier.

In niri I can do

input {
  keyboard {
    xkb {
      layout "us"
      variant "altgr-intl"
      options "compose:rctrl"
    }
  }
  ...

and wev sees rctrl as Multi_key (key 105), but it's not in valid modifiers in Configuration:-Input.md and niri doesn't recognize it for mapping.

YaLTeR@Niri said: If this modifier is recognized by xkb as such, then it needs to be added in Smithay for niri to support this kind of binding YaLTeR/niri#1405

Would it be possible to add Multi_key to the valid modifiers to Smithay?