Allow using built-in names for variables, push warnings instead

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

@@ -0,0 +1,2 @@
+func test():
+	var abs = "This variable has the same name as the built-in function."

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

@@ -0,0 +1,9 @@
+GDTEST_OK
+>> WARNING
+>> Line: 2
+>> SHADOWED_GLOBAL_IDENTIFIER
+>> The local variable 'abs' has the same name as a built-in function.
+>> WARNING
+>> Line: 2
+>> UNUSED_VARIABLE
+>> The local variable 'abs' is declared but never used in the block. If this is intended, prefix it with an underscore: '_abs'