Add static analysis rule to detect SendMessage call with invalid method name

Unity provides an API called [SendMessage](https://docs.unity3d.com/ScriptReference/GameObject.SendMessage.html) that allows invocation of a method by name.

There is no validation on whether the method even exists, and so it would make for an ideal candidate for static code analysis.

A nice validation would be to warn in case of usage of SendMessage with an invalid method name.