New: Add operator-assignment rule (fixes #1420)

This implements the first rule from WebStorm Inspectors (rollup) #667.
The `isCommutative` and `isNonCommutative` functions only check the binary operators that also have a shorthand form. Those with no equivalent (`<`, `>`, `<=`, `>=`, `==`, `!=`, `===`, `!==`, `&&`, `||`) should fall through both tests. However, this did prompt me to add those operators to the valid tests to make sure it doesn't report any false positives. Amended and pushed.