GDScript: Suppress unused constant warning with underscore

The warning message mentions that local constants prefixed with `_` does
not generate the warning. This commit actually implements this warning
suppression.

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

@@ -0,0 +1,4 @@
+func test():
+	const UNUSED = "not used"
+
+	const _UNUSED = "not used, but no warning since the constant name starts with an underscore"