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

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

What rule do you want to change?

no-useless-escape

Does this change cause the rule to produce more or fewer warnings?

Fewer, by opt-in

How will the change be implemented? (New option, new default behavior, etc.)?

A new option will be added.

Please provide some example code that this change will affect:

/* eslint no-useless-escape: [2, {"ignoreCharClasses": true}] */

var foo = /[\.\*]/; // a character class matching a dot or a star character

What does the rule currently do for this code?

It reports the escapes as useless, since they can be removed without affecting the code.

What will the rule do after it's changed?

With the new ignoreCharClasses: true option, the rule will allow characters in regular expressions to be uselessly escaped, if that character is sometimes a special character in regular expressions. For example, with the new option enabled, \* will be allowed in all regular expressions, even though escaping is only required for * in a character class. The default is ignoreCharClasses: false.

What changes did you make? (Give an overview)

This adds an ignoreCharClasses option to no-useless-escape, in case people would prefer to always escape certain characters in regular expressions rather than worrying about whether the character is in a character class.

TODO: Add documentation for the new option

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

Do not merge yet; this depends on #7425. Specifically, see #7425 (comment) for discussion about this option.

We still need to decide whether it's worth adding this option. I'm not opposed to adding it if a lot of people are interested in it, but I'm not championing it at the moment; I think maybe we should release #7425 and see how many people would actually want this option. I'm making this PR now in case we decide we want to get this change into the same release as #7425.