checker: don't disallow defining methods on interfaces (#8335)

vlib/v/tests/methods_on_interfaces_test.v

@@ -0,0 +1,20 @@
+struct Cat {
+	breed string
+}
+
+interface Animal {
+	breed string
+}
+
+fn (a Animal) info() string {
+	return "I'm a ${a.breed} ${typeof(a).name}"
+}
+
+fn new_animal(breed string) Animal {
+	return &Cat{ breed }
+}
+
+fn test_methods_on_interfaces() {
+	mut a := new_animal('persian')
+	assert a.info() == "I'm a persian Animal"
+}