feat(i18n): proper multilingual input

Use fcitx5 to switch between different inputs. Installed mozc-ut for
Japanese input with the bepo layout, as well as fcitx5-chinese to load
table-defined layouts for the IPA/X-SAMPA input method.

Currently, mozc_server and fcitx5 need to be launched by the
compositor (only Hyprland for now), auto-launching from module options
needs to be implemented in the future.

system/i18n/default.nix

@@ -1,5 +1,6 @@
 {
   imports = [
+    ./input.nix
     ./locale.nix
   ];
 }