No more `Char`

[justinwoo]

I really don't think Char helps here in any way, since that's not even a thing in JS and now we have some ambiguity between CodeUnits and CodePoints. Can we be rid of this and properly use String types wherever we have Strings?

[anttih]

I just noticed while glancing over the implementation that CodePoints uses charAt in anyChar which deals with code units instead of code points. So either it should return a CodePoint or a String as you suggested.

[hdgarrood]

I'd prefer to make a decision on purescript/purescript#3662 first.

[github-actions]

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