feat: Integrate custom prop library and sandbox functionality

- Added support for custom props in the sandbox, including loading, saving, and rendering.
- Implemented `PropLibrary` for managing custom prop entries and scanning for `.prop` files.
- Introduced `PropVisuals` for shared visual and collision representation of custom props.
- Enhanced `StageSpawner` to handle spawning of custom props with layered visuals and collision hulls.
- Created `MissingPropPlaceholder` to manage missing custom props during scene reloads.
- Updated thumbnail generation to support custom props with unique keys based on file paths.
- Added tests for custom prop lifecycle, authored visual values, and missing prop handling.
- Refactored existing code to accommodate new prop functionalities and ensure backward compatibility.

tests/test_prop_editor_thumb.gd

@@ -47,9 +47,9 @@ func _test_prop_file_keys() -> void:
 	f.close()
 	var key: String = cache.prop_file_key(path)
 	var mtime: int = FileAccess.get_modified_time(path)
-	check("prop_file_key format", key == "prop_thumb_key_test_%d" % mtime)
+	check("prop_file_key includes file namespace and mtime", key.begins_with("file_") and key.ends_with("_%d" % mtime))
 	check("prop_file_png uses PROP_DIR", cache.prop_file_png(path) == "user://thumbnails/props/%s.png" % key)
-	check("prop_file_key missing file still formats", cache.prop_file_key("res://nope.prop") == "nope_0")
+	check("prop_file_key missing file still formats", cache.prop_file_key("res://nope.prop").begins_with("file_") and cache.prop_file_key("res://nope.prop").ends_with("_0"))
 	DirAccess.remove_absolute(path)