feat!: no-invalid-regexp make allowConstructorFlags case-sensitive

[JoshuaKGoldberg]

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:
What change do you want to make (place an "X" next to just one item)?

[x] Generate more warnings
[ ] Generate fewer warnings
[ ] Implement autofix
[ ] Implement suggestions

How will the change be implemented (place an "X" next to just one item)?

[ ] A new option
[x] A new default behavior
[ ] Other

Please provide some example code that this change will affect:

const ESLint = require("eslint");
console.log(
  new ESLint.Linter().verify('RegExp(".", "A");', {
    rules: { "no-invalid-regexp": [1, { allowConstructorFlags: ["a"] }] },
  })
);

What does the rule currently do for this code?

Treats all allowed constructor flags as case-insensitive.

What will the rule do after it's changed?

They're now case-sensitive, as per the issue.

Adds a unit test for code being lowercase vs. rule config being uppercase, and vice-versa.

Fixes #16574.

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[nzakas]

The code LGTM. Can you also add a note in the docs indicating that this option is case sensitive?

[nzakas]

LGTM. Will let this sit until we're ready to merge v9 PRs.

[github-actions]

Hi everyone, it looks like we lost track of this pull request. Please review and see what the next steps are. This pull request will auto-close in 7 days without an update.

[mdjermanovic]

LGTM, thanks!