Fix String.foldr bug that causes infinite loop

[arowM]

String.all, String.any, and functions using String.foldr have a bug that can cause infinite loop under certain conditions.

SSCCE

-- `String.toList` is implemented with `String.foldr`
String.right 1 "foobar😈" |> String.toList
--> (causes infinite loop)

-- The following code, which determines whether the last character of a string is a space, is not uncommon:
String.right 1 "foobar😈" |> String.any (\c -> c == ' ')
--> (causes infinite loop)

String.right 1 "foobar😈" |> String.all (\c -> c /= ' ')
--> (causes infinite loop)

Cause of the bug

Functions such as foldr do not expect that only part of a surrogate pair is given as an argument.

var _String_foldr = F3(function(func, state, string)
{
	var i = string.length;
	while (i--)
	{
		var char = string[i];
		var word = string.charCodeAt(i);
		if (0xDC00 <= word && word <= 0xDFFF)
		{
			// This is harmful.
			i--;
			char = string[i] + char;
		}
		state = A2(func, __Utils_chr(char), state);
	}
	return state;
});

Hence, i in the above code jumps over 0 and becomes a negative value, resulting in an infinite loop.

[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.