Add new Style/SwapValues cop

[fatkodima]

This cop enforces the use of shorthand-style swapping of 2 variables.

# bad
tmp = x
x = y
y = tmp

# good
x, y = y, x

This will be mostly useful for newcomers from other languages into ruby, because when you used it once you will always remember about its existence.

[bbatsov]

Great idea! Seems that we should also add this to the style guide.

[bbatsov]

The wording here is a bit misleading, as the cop is safe and only the auto-correct is unsafe.

[fatkodima]

Added link to the style guide (rubocop/ruby-style-guide#853) and changed wording.

[bbatsov]

Thanks!

[pvdb]

There's an issue #10394 with this cop that I just ran into.