Drop Data.Char.Unicode isDigit dependency

[paulyoung]

@natefaubion pointed out this dependency. We should probably just inline this:

-- | Selects ASCII digits, i.e. `0..9`.
isDigit :: Char -> Boolean
isDigit c = let diff = (toCharCode c - toCharCode '0')
            in diff <= 9 && diff >= 0

https://github.com/purescript-contrib/purescript-unicode/blob/de173a882515bad669949a1dceccb49c9f96d48a/src/Data/Char/Unicode.purs#L366-L369