Add new `ArrayPushSingle` cop to replace `a.push(x)` with `a << x`

[amomchilov]

Closes #431

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[ydakuka]

Is this related to https://github.com/rubocop/rubocop/issues/12053 ?

[amomchilov]

@ydakuka I hadn't seen that, but I guess it's pretty similar!

I think two separate cops would be good here. One to turn a.push(x) into a << x, and another to take a series of a << xs (or pushes) and turn them into one.