GDScript: Be more lenient with identifiers

- Allow identifiers similar to keywords if they are in ASCII range.
- Allow constants to be treated as regular identifiers.
- Allow keywords that can be used as identifiers in expressions.

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

@@ -1,5 +1,12 @@
+extends Node
+
 func test():
 	var port = 0 # Only latin characters.
 	var pοrt = 1 # The "ο" is Greek omicron.
 
 	prints(port, pοrt)
+
+# Do not call this since nodes aren't in the tree. It is just a parser check.
+func nodes():
+	var _node1 = $port # Only latin characters.
+	var _node2 = $pοrt # The "ο" is Greek omicron.