add Belt.String

[bobzhang]

see #2758
This PR is just throwing some ideas so that the least controversial API could be added.
It would be nice that people could take the ownership of the PR

[lpil]

What would need to be added to the above PR for it to be merged?

Given it makes heavy use of external functions how would it be used when compiling to native?

[bobzhang]

it will be compiled into native via conditional compilation

[namenu]

I wonder what's left to move this forward?

IF native related issues aren't valid anymore, @bloodyowl 's String binding seems an excellent candidate to have.
(it contains some ES8+, though)

I know there has been discussion about SubString type, but that's tangential when the only target is JS, right?

As s ReScript user, with -open Belted, what I want most is type-safe and ergonomic Belt.String, not Js.String2. 🥺

[bobzhang]

@namenu Indeed, we should have this module in Belt. It is not done yet is mostly because of lack of time, contributions are welcome! (We also need some one to help review the code)

[cknitt]

Superseded by the String module in the new ReScript "Core" standard library.

https://github.com/rescript-association/rescript-core/blob/main/src/Core__String.res