Suggested cop: prefer `first`/`drop`/`last` over `[]` with particular kinds of ranges

[amomchilov]

There's a bunch of overloads of #slice/#[], many of which are technically nilable, but don't return nil in practice.

We can't fix that in the general case, but of a lot uses with ranges start from 0 (or ending in -1 can be replaced by more dedicated methods like first/drop/last, which aren't nilable.

A challenge here is that the APIs aren't consistent between Array and String (and other types), and there's no way to only make the change for particular types.

These examples assume:

a = Array(0..9)
s = Array("a".."z").join

Replace with first

Applicable for Array, but requires a patch from ActiveSupport for String.

-T.must(s[0..5])
-T.must(s[ ..5])
+s.first(6)

-T.must(s[0...5])
-T.must(s[ ...5])
-T.must(s[0, 5])
+s.first(5)

Replace with last

Applicable for Array, but not to String.

-T.must(a[5..-1])
-T.must(a[5..  ])
-T.must(a[5... ])
+a.drop(5)

Replace with dup

Really, these likely just be s, but we can't be sure that the callers weren't relying on the result being a new copy of the string, so we'd conservatively need to dup it.

-T.must(s[0..])
-T.must(s[0...])
-T.must(s[..-1])
s.dup