GDScript: Infer type with string format operator

If the left value type is known to be String, assume the format operator
(`%`) will return a string, since it works with any type in the right
hand side. This is also used by type inference even if the right hand
type is unknown at compile time.

modules/gdscript/tests/scripts/analyzer/features/infer_type_on_string_format.gd

@@ -0,0 +1,6 @@
+# GH-88082
+
+func test():
+	var x = 1
+	var message := "value: %s" % x
+	print(message)

modules/gdscript/tests/scripts/analyzer/features/infer_type_on_string_format.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+value: 1