Add support for Unicode identifiers in GDScript

This is using an adapted version of UAX#31 to not rely on the ICU
database (which isn't available in builds without TextServerAdvanced).
It allows most characters used in diverse scripts but not everything.

modules/gdscript/gdscript_tokenizer.h

@@ -224,6 +224,9 @@ private:
 	char32_t indent_char = '\0';
 	int position = 0;
 	int length = 0;
+#ifdef DEBUG_ENABLED
+	Vector<String> keyword_list;
+#endif // DEBUG_ENABLED
 
 #ifdef TOOLS_ENABLED
 	HashMap<int, CommentData> comments;
@@ -239,6 +242,10 @@ private:
 	void _skip_whitespace();
 	void check_indent();
 
+#ifdef DEBUG_ENABLED
+	void make_keyword_list();
+#endif // DEBUG_ENABLED
+
 	Token make_error(const String &p_message);
 	void push_error(const String &p_message);
 	void push_error(const Token &p_error);