Make it possible to opt out of merging behavior with `experimentalParseClassName`

[dcastil]

Request coming from problem stated in #493.

[dcastil]

This will be addressed in the next major tailwind-merge version. I added the property isExternal to the returned object from experimentalParseClassName that allows to opt out of merging behavior and treat a class like a non-Tailwind class.

I did this as part of the the Tailwind CSS v4 support PR because that behavior is necessary for the new prefix syntax in Tailwind CSS v4.

Done in 380c977.

[github-actions]

This was addressed in release v3.0.0.