Add string/path utility nodes (PathJoin, StringFormat, StringExtract, StringSwitch)

Four new nodes that collapse common multi-node chains for path construction,
string templating, substring extraction, and boolean selection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

__init__.py

@@ -1,4 +1,14 @@
-from .json_loader_dynamic import NODE_CLASS_MAPPINGS, NODE_DISPLAY_NAME_MAPPINGS
+from .json_loader_dynamic import (
+    NODE_CLASS_MAPPINGS as _json_class_mappings,
+    NODE_DISPLAY_NAME_MAPPINGS as _json_display_mappings,
+)
+from .string_utils import (
+    NODE_CLASS_MAPPINGS as _string_class_mappings,
+    NODE_DISPLAY_NAME_MAPPINGS as _string_display_mappings,
+)
+
+NODE_CLASS_MAPPINGS = {**_json_class_mappings, **_string_class_mappings}
+NODE_DISPLAY_NAME_MAPPINGS = {**_json_display_mappings, **_string_display_mappings}
 
 WEB_DIRECTORY = "./web"