JS: Sharpen up EnumerationRegExp

[asgerf]

The class EnumerationRegExp is intended to capture the idea of a regexp that acts as a hardcoded list of values to check, e.g. /^(foo|bar|baz)$/ but with some leeway for choice, like /^(found/recogni[sz]ed)$/.

However, the implementation used an exists that was probably meant to be forall or forex, which means it was way too loose in what it accepted. This gave rise to some invalid barrier guards, which is not caught by any existing test nor by DCA, but I have a simple tests now. This PR makes it more restrictive.

Regexp-based barrier guards are primarily the responsibility of SanitizerRegExpTest, but EnumerationRegExp gives rise to MembershipCandidate which gives rise to barrier guards, so we have two ways for regexp checks to become barrier guards. The ones from EnumerationRegExp should however be a subset of those from SanitizerRegExpTest and previously they weren't.

[Copilot]

PR Overview

This PR addresses an overly permissive regular expression in the EnumerationRegExp class, making it more restrictive to correctly form barrier guards. Key changes include:

• Updating change notes to reflect the bug fix.
• Adding a new test file to verify the improved regexp logic.

Reviewed Changes

File	Description
javascript/ql/src/change-notes/2025-02-28-membership-regexp-test.md	Updates for bug fix documentation and messaging
javascript/ql/test/library-tests/TaintTracking/regexp-sanitiser.js	Addition of a test case for regexp-based sanitizer behavior

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