JS: fix FP related to block-level flow type annotations

[erik-krogh]

Block-level flow type annotations were falsely flagged as useless-expressions.

E.g:

  ...
  default:
     (type: empty); // flagged by `js/useless-expression`
     throw new Error(`Received invalid event property type ${type}`);
  ...

Currently the check is purely syntactical by inspecting the tokens to recognize the (e: type) pattern.
It doesn't seem like a nice way to do it, but it was the best I could get to work.

An evaluation shows that plenty of false positives are removed, and performance seems to be OK.

[max-schaefer]

Nice! That's actually simpler than I thought. Could you add a test?