feat: add HTML expectation

[kbond]

I've created a new zenstruck/assert-html package which is like a plugin for this library to make html assertions. ~I've extracted this code from zenstruck/browser and will create a followup PR there to utilize this new library.~ edit: doing this created more complexity than I liked. Additionally, this new package will allow https://github.com/zenstruck/mailer-test/issues/1.

Some notes:

1. I created as a separate package because of the mink/dom-crawler deps.
2. At some point I'll move the maintenance here and zenstruck/assert-html will be a readonly sub-tree split.
3. There is quite a bit of duplication between the mink driver in the plugin and browser which is unfortunate. When/if it becomes a problem, I can move to a separate package.

TODO:

• [ ] wait for 1.0 release of zenstruck/assert-html

[kbond]

I actually have an alternative system I'm cooking up. Stay tuned :)