Fix regex flags being ignored in regex body matching

[theluckystrike]

withBody() with a regular expression drops the regex flags. RegexBodyMatcher only keeps regex.source and rebuilds the RegExp without flags, so a rule like .withBody(/"username": "test"/i) doesn't match a body with different casing. The rule explanation also leaves the flags out.

This keeps regex.flags next to the source and uses it when matching and in explain(), the same way RegexPathMatcher already does since 04c5320.

Added a test in body-matching.spec.ts that sends an uppercase body to the existing /gi rule. It fails on main and passes with this change. npm run build:src and npm run test:node pass locally (836 passing).

[CLAassistant]

All committers have signed the CLA.

[pimterry]

Very nice, thanks @theluckystrike! Merged.

By the way, in case you're not aware, HTTP Toolkit Pro is free for all contributors to all https://github.com/httptoolkit repos. If that would be useful, just let me know your email (either here or by sending a message to opensource @ httptoolkit.com) and I'll set you up with an account.

[theluckystrike]

Thanks for the quick merge, and for the Pro offer. That's kind of you. I'll get in touch if I end up needing it.