New lint cop to check mistypes in += or -=

[fatkodima]

Inspired by https://rules.sonarsource.com/ruby/type/Bug/RSPEC-2757 and https://www.viva64.com/en/w/v588/

I think many people did mistakes like the following

x =+ 2

[Drenmi]

I guess the case of x =- 2 is more likely to have false positives, because it could be x = -2, but the space went in the wrong place. 🙂 Either way, I think this is a solid candidate for a Lint cop.

[bbatsov]

I also think such a cop is a good idea!