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/inventory/inventory_ui.tscn

@@ -0,0 +1,31 @@
+[gd_scene load_steps=3 format=2]
+
+[ext_resource path="res://addons/escoria-core/game/scenes/inventory/inventory_ui.gd" type="Script" id=1]
+[ext_resource path="res://addons/escoria-core/ui_library/inventory/esc_inventory_container.gd" type="Script" id=3]
+
+[node name="inventory_ui" type="PanelContainer"]
+margin_right = 600.0
+margin_bottom = 175.0
+script = ExtResource( 1 )
+__meta__ = {
+"_edit_use_anchors_": false
+}
+inventory_ui_container = NodePath("ScrollContainer/GridContainer")
+
+[node name="ScrollContainer" type="ScrollContainer" parent="."]
+margin_left = 7.0
+margin_top = 7.0
+margin_right = 593.0
+margin_bottom = 168.0
+size_flags_horizontal = 3
+size_flags_vertical = 3
+
+[node name="GridContainer" type="GridContainer" parent="ScrollContainer"]
+margin_right = 586.0
+margin_bottom = 161.0
+size_flags_horizontal = 3
+size_flags_vertical = 3
+custom_constants/vseparation = 16
+custom_constants/hseparation = 16
+columns = 4
+script = ExtResource( 3 )