Add String methods

[petele]

No description provided.

[ddbeck]

Following up on the Intl discussion elsewhere, I'm suggesting dropping the Intl-related methods in line comments.

Also: @Elchi3, you and I talked about this (and Number and some others). I think you rightly pointed out that at least some string methods ought to have their own features (e.g., at()), for symmetry with the Array features. Should we pull that stuff out now? Or would it make more sense to split it up in a separate PR after merging?

[Elchi3]

Also: @Elchi3, you and I talked about this (and Number and some others). I think you rightly pointed out that at least some string methods ought to have their own features (e.g., at()), for symmetry with the Array features. Should we pull that stuff out now? Or would it make more sense to split it up in a separate PR after merging?

I think I would pull that out now and possibly figure out in this PR what we want the "String (initial support)" feature to be here (analog to features/array.yml).

[petele]

Based on feedback, and to somewhat mirror Array, I split string into multiple features, and create a new String group to which they all belong.

[ddbeck]

I've suggested a number of changes, mostly to names and descriptions. I didn't apply them because a few file renames are needed. Thank you!

Also, @Elchi3, optionally if you want to give this a once-over to compare to the Array features, I'd appreciate it.

[Elchi3]

LGTM, great work 👍