Store InventoryItems as separate resource files

[wjt]

Previously we defined InventoryItem resources inline in every scene that
uses them; and we hardcoded the world texture, HUD texture, and colour
for each in the script.

Turn these into properties of the resource. Create 3 on-disk resources
for the memory, imagination, and spirit threads. Add migration code to
collectible_item.gd which updates the property so that loading & saving
a scene using this script does the migration. Leave the migration code
in-place so that as out-of-tree quests are merged we can easily update
them.

Change the inventory storage to refer to those, rather than saving just
the item type name and reconstructing resources on load.

I think this will make it easier to add collectibles which are not
threads in future, and also to remember which threads have been
collected when loading or replaying a quest, though I haven't attempted
to do that here.

[github-actions]

Play this branch at https://play.threadbare.game/branches/endlessm/wjt/store-inventoryitems-as-separate-resource-files/.

(This launches the game from the start, not directly at the change(s) in this pull request.)

[wjt]

Rebased and re-rewrote all the scenes.

I think this change makes sense. It does increase the chance of someone accidentally editing the text of one of the three threads rather than making a bespoke one. However we don't currently support collecting anything except these three items. And actually I think this is a useful step towards more generic collectibles.

[wjt]

Personally I would recommend tagging a release before merging this and #2693!