Universalize UID support in all resource types

Ensures all resource types support UIDs in a project.

This is required to fix:
* Scripts and many other resource types can't be referenced by UID and when refactored the references are lost.
* Path export properties can't use UID for unsupported types.
* Refactoring problems when files are moved outside the editor (this PR effectively fixes it).
* Editor properly refreshing paths if they changed externally while opened (as example, git update).
  This needs to be addressed in a subsequent PR, but this one effectively sets the prerequisites.

Resource types that do not support UID will get a .uid file appended to them (this includes .gd, .gdshader, .gdextension, etc. files).

doc/classes/ResourceFormatLoader.xml

@@ -57,6 +57,7 @@
 			<return type="int" />
 			<param index="0" name="path" type="String" />
 			<description>
+				Should return the unique ID for the resource associated with the given path. If this method is not overridden, a [code].uid[/code] file is generated along with the resource file, containing the unique ID.
 			</description>
 		</method>
 		<method name="_handles_type" qualifiers="virtual const">