Make Char.isLower and Char.isUpper Unicode-aware (2nd try)

[Janiczek]

⚠️ This is just #970 with a fix for the imports. The patch, if applied, should now compile instead of complaining about == and /=.

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This allows people with non-ASCII alphabets work with Char.isLower and Char.isUpper. Uses toUpper and toLower underneath, which use Javascript's String.prototype.toLower/UpperCase().

The second condition in the functions is there to distinguish between characters that have an upper/lower-case pairing, and those that don't ('0' == Char.toLower '0' but we don't want isLower '0' to be true).

[github-actions[bot]]

Thanks for suggesting these code changes. To set expectations:

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