Add `string sub --end`

[zx8]

Would be neat if string sub --length accepted negatives indexes, just like string sub --start, e.g.

$ string sub --length -2 -- foobar
foob

AFAIK, the only way to achieve this at the moment is:

$ set s foobar
$ string sub --length (math (string length -- $s) - 2) -- $s
foob

(Also, feel like this option should be (re?)named --end to match --start)

[faho]

That's an entirely different thing?

Like... with a length of 3, you'd go from a start of e.g. 2 to an end of 5.

Which is an okay thing to add, I just don't see why we should remove --length for it.

[faho]

E.g. string sub --start 2 --length 3 foobar prints "oob" (a 3-long string starting with the second character).

string sub --start 2 --end 3 foobar should print either oo or just o. (Arguably it'd be better to include the end, like we include the start, so oo)

[zx8]

Aha, misunderstood --length, got it!