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/warnings/confusable_identifier.gd

@@ -0,0 +1,5 @@
+func test():
+	var port = 0 # Only latin characters.
+	var pοrt = 1 # The "ο" is Greek omicron.
+
+	prints(port, pοrt)

modules/gdscript/tests/scripts/parser/warnings/confusable_identifier.out

@@ -0,0 +1,6 @@
+GDTEST_OK
+>> WARNING
+>> Line: 3
+>> CONFUSABLE_IDENTIFIER
+>> The identifier "pοrt" has misleading characters and might be confused with something else.
+0 1