Add deprecation warning when String#first and String#last receive neg…

[gmcgibbon]

…ative integers

Summary

This adds a deprecation warning when callingString#first and String#last with negative integer arguments. The current behaviour is the following:

"hello".last(-1) #=> "ello"
"hello".first(-1) #=> "hell"
"hello".last(-6) #=> nil
"hello".first(-6) #=> ""

As you can see, it doesn't really line up with Array's behaviour:

["h", "e", "l", "l", "o"].last(-1) #=> ArgumentError (negative array size)
["h", "e", "l", "l", "o"].first(-1) #=> ArgumentError (negative array size)

See #29105 for the discussion.

[rafaelfranca]

Use 6.1 here.