Implement Resource UIDs

* Most resource types now have unique identifiers.
* Applies to text, binary and imported resources.
* File formats reference both by text and UID (when available). UID always has priority.
* Resource UIDs are 64 bits for better compatibility with the engine.
* Can be represented and used textually, example `uuid://dapwmgsmnl28u`.
* A special binary cache file is used and exported, containing the mappings.

Example of how it looks:

```GDScript
[gd_scene load_steps=2 format=3 uid="uid://dw86wq31afig2"]

[ext_resource type="PackedScene" uid="uid://bt36ojelx8q6c" path="res://subscene.scn" id="1_t56hs"]
```

GDScript, shaders and other special resource files can't currently provide UIDs, but this should be doable with special keywords on the files.
This will be reserved for future PRs.

2 files changed, 5 insertions, 0 deletions

diff --git a/core/object/class_db.cpp b/core/object/class_db.cpp
index e2db5918e3..f871dd464a 100644
--- a/core/object/class_db.cpp
+++ b/core/object/class_db.cpp
@@ -1496,6 +1496,10 @@ void ClassDB::get_resource_base_extensions(List<String> *p_extensions) {
 	}
 }
 
+bool ClassDB::is_resource_extension(const StringName &p_extension) {
+	return resource_base_extensions.has(p_extension);
+}
+
 void ClassDB::get_extensions_for_type(const StringName &p_class, List<String> *p_extensions) {
 	const StringName *K = nullptr;
 
diff --git a/core/object/class_db.h b/core/object/class_db.h
index fd574fd2d8..3a84e9ab38 100644
--- a/core/object/class_db.h
+++ b/core/object/class_db.h
@@ -396,6 +396,7 @@ public:
 	static void add_resource_base_extension(const StringName &p_extension, const StringName &p_class);
 	static void get_resource_base_extensions(List<String> *p_extensions);
 	static void get_extensions_for_type(const StringName &p_class, List<String> *p_extensions);
+	static bool is_resource_extension(const StringName &p_extension);
 
 	static void add_compatibility_class(const StringName &p_class, const StringName &p_fallback);