Add dozens of new integration tests to the GDScript test suite

This also ignores `.out` files in the file format static checks.

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

@@ -0,0 +1,11 @@
+func test():
+	const _TEST = 12 + 34 - 56 * 78
+	const _STRING = "yes"
+	const _VECTOR = Vector2(5, 6)
+	const _ARRAY = []
+	const _DICTIONARY = {"this": "dictionary"}
+
+	# Create user constants from built-in constants.
+	const _HELLO = PI + TAU
+	const _INFINITY = INF
+	const _NOT_A_NUMBER = NAN