Unify lpad, rpad and cpad and move to external package

[dpsanders]

lpad, rpad and cpad would seem to be good candidates to be unified in a pad function.

There are a lot of such string manipulation functions that feel like they should be moved from Base to a string manipulation package.

[KristofferC]

I don't see any benefit in moving short, pure, julia code that is generally useful to a package.

[quinnj]

I could see us making an official Strings module that would still be apart of Base/StdLib, but yeah, a lot of these functions are usually expected to be in the "default" language.

[StefanKarpinski]

We could always just use https://api.left-pad.io/

[KristofferC]

I don't think this will be moved out of this repo. Perhaps to a String stdlib as part of #25416