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.out

@@ -1,6 +1,10 @@
 GDTEST_OK
 >> WARNING
->> Line: 3
+>> Line: 5
+>> CONFUSABLE_IDENTIFIER
+>> The identifier "pοrt" has misleading characters and might be confused with something else.
+>> WARNING
+>> Line: 12
 >> CONFUSABLE_IDENTIFIER
 >> The identifier "pοrt" has misleading characters and might be confused with something else.
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