feat: introduce escoria-ui-keyboard-9verbs to verify register_custom_input_handler() API

This commit was started by copying `addons/escoria-ui-9verbs` as the
basis for `addons/escoria-ui-keyboard-9verbs`. It was then amended to
wire up keyboard shortcuts for each verb using the new
`register_custom_input_handler()` API.

Note that `addons/escoria-ui-keyboard-9verbs/input_map.gd` introduces
one action per verb and provides a `add_actions_to_input_map()`
function to add the actions to the `InputMap`. The `_process_input()`
function in `game.gd` is responsible for mapping each action to the
appropriate call to `verbs_menu.on_action_selected()`.

addons/escoria-ui-keyboard-9verbs/plugin.gd

@@ -0,0 +1,18 @@
+# Plugin script to initialize Escoria simple mouse UI
+tool
+extends EditorPlugin
+
+
+# Register UI
+func _enter_tree() -> void:
+	call_deferred("_register")
+
+
+# Deregister UI
+func _exit_tree() -> void:
+	escoria.deregister_ui("res://addons/escoria-ui-keyboard-9verbs/game.tscn")
+
+
+# Register UI with Escoria
+func _register():
+	escoria.register_ui("res://addons/escoria-ui-keyboard-9verbs/game.tscn")