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]

Thanks for suggesting these code changes. To set expectations:

• Pull requests are reviewed in batches, so it can take some time to get a response.
• Smaller pull requests are easier to review. To fix nine typos, nine specific issues will always go faster than one big one. Learn why here.
• Reviewers may not know as much as you about certain situations, so add links to supporting evidence for important claims, especially regarding standards for CSS, HTTP, URI, etc.

Finally, please be patient with the core team. They are trying their best with limited resources.