GDExtension: add an interface for loading extra documentation

Adds two new GDExtension interface methods:
 - `editor_help_load_xml_from_utf8_chars`
 - `editor_help_load_xml_from_utf8_chars_and_len`

Both of these methods parse the XML passed into an extra documentation
container which, when needed, is merged into the main doc container.

Co-Authored-By: Rémi Verschelde <rverschelde@gmail.com>

editor/doc_tools.h

@@ -56,6 +56,7 @@ public:
 
 	Error _load(Ref<XMLParser> parser);
 	Error load_compressed(const uint8_t *p_data, int p_compressed_size, int p_uncompressed_size);
+	Error load_xml(const uint8_t *p_data, int p_size);
 };
 
 #endif // DOC_TOOLS_H