[RFC]: add `string/base/slice`

[kgryte]

This task should implement a functional API for slicing a string based on UTF-16 code units.

The behavior should match the built-in String.prototype.slice. However, all arguments should be required:

slice( str, start, end )

Related: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/string/base/for-each

Time estimate without AI: 4hrs
Time estimate with AI: 2-3hrs

[headlessNode]

PR: stdlib-js/stdlib#5395
Time till opening PR: 1hr 3mins
Loom recording: link

[headlessNode]

Notes:

• This was an easy one. With AI, I'm sure, time till opening PR for this one would have been at most 45mins.

[headlessNode]

Post Review Notes:

• Around 5 mins spent post review on refactoring.