feat: add option allowEscape to no-misleading-character-class rule

[fasttime]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[X] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Fixes #15080

What changes did you make? (Give an overview)

This PR adds an option to allow escape sequences in regexp character classes to the no-misleading-character-class rule. The new option only has effect on nodes whose source location can be statically determined with our tools. These are the same nodes that report granular errors:

• regular expression literals
• string literals as first arguments of the RegExp constructor
• template literals without expressions as first arguments of the RegExp constructor

Is there anything you'd like reviewers to focus on?

I would like to fine tune this rule to allow only escape sequence that are not confusing, and I'm not sure I've got the right balance. For example /[\👍]/ contains an escape sequence where the backslash just escapes the next character without changing its value. I think that those escape sequences should be still forbidden even when the new option is set. Other cases are less trivial.

In the current implementation, the new option will allow any escape sequences except those where the character being escaped is prepended by one or more backslashes. If one or more of the characters in a group are escaped, the group will be allowed.

Node	Status
/[A\u0301]/	allowed, second character in class is escaped
/[👨\u200d👩\u200d👦]/u	allowed, ZWNJ character is escaped
/[\n̅]/	allowed, \n is a non-trivial escape sequence
/[\e̅]/	not allowed, \e is e

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[mdjermanovic]

/[\n̅]/ allowed, \n is a non-trivial escape sequence
/[\e̅]/ not allowed, \e is e

These two look like the same kind of possible error this rule targets - the sequence appears as a single character - so perhaps both should not be allowed?

[fasttime]

/[\n̅]/ allowed, \n is a non-trivial escape sequence
/[\e̅]/ not allowed, \e is e

These two look like the same kind of possible error this rule targets - the sequence appears as a single character - so perhaps both should not be allowed?

Maybe we could require that both characters in n̅ are escaped? I'm not sure, because the problem here seems to lie in the unescaped combining character itself rather than the grouping of characters, and the rule does not flag those characters everywhere. This means that character classes like /[̳]/ or /[\n\u0305̳abc]/ will remain allowed despite looking confusing.

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[fasttime]

Perhaps we could allow only sequences that consist exclusively of ASCII characters. So /[\n\u0305]/ would be allowed, but /[\n̅]/ or /[\n\̅]/ would be not. What do you think @mdjermanovic?

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[fasttime]

I think we can require that combining mark characters in a character class are only allowed when written as escape sequences; which is to say regardless of the previous character. This would be the most immediate solution to always disallow /[\n̅]/ and /[\u006e̅]/ along with /[\e̅]/. I'll work on an update.

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