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/fonts/caslonantique.tres

@@ -0,0 +1,7 @@
+[gd_resource type="DynamicFont" load_steps=2 format=2]
+
+[ext_resource path="res://addons/escoria-ui-keyboard-9verbs/fonts/caslonantique.ttf" type="DynamicFontData" id=1]
+
+[resource]
+size = 21
+font_data = ExtResource( 1 )