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/tests/scripts/parser/features/unicode_identifiers.out

@@ -0,0 +1,14 @@
+GDTEST_OK
+փորձարկում test
+امتحان test
+পরীক্ষা test
+тест test
+जाँच test
+기준 test
+测试 test
+テスト test
+試験 test
+പരീക്ഷ test
+ทดสอบ test
+δοκιμή test
+true