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/features/allow_id_similar_to_keyword_in_ascii.gd

@@ -0,0 +1,3 @@
+func test():
+	var P1 = "ok" # Technically it is visually similar to keyword "PI" but allowed since it's in ASCII range.
+	print(P1)

modules/gdscript/tests/scripts/parser/features/allow_id_similar_to_keyword_in_ascii.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+ok

modules/gdscript/tests/scripts/parser/features/allowed_keywords_as_identifiers.gd

@@ -0,0 +1,16 @@
+func test():
+	# The following keywords are allowed as identifiers:
+	var match = "match"
+	print(match)
+
+	var PI = "PI"
+	print(PI)
+
+	var INF = "INF"
+	print(INF)
+
+	var NAN = "NAN"
+	print(NAN)
+
+	var TAU = "TAU"
+	print(TAU)

modules/gdscript/tests/scripts/parser/features/allowed_keywords_as_identifiers.out

@@ -0,0 +1,6 @@
+GDTEST_OK
+match
+PI
+INF
+NAN
+TAU