Make `set` keyword optional and include lint to suggest avoiding it.

The `set` and `let` keywords are very similar and easy to confuse, leading to accidental shadowing sometimes. In addition, `let` is already sufficient to disambiguate declaration/binding from assignment, so `set` doesn't provide any actual value syntactically. We can leave the keyword in but make it optional in the parser, and then later follow up with a lint rule about it.