feat: accept many selectors in assertSeeIn() and assertNotSeeIn()

[Amoifr]

Closes #16.

assertSeeIn() and assertNotSeeIn() now also take an iterable of selectors, exactly as sketched in the issue. Every selector must contain the text for the positive assertion, and none may for the negative one.

assertNotSeeIn() was not part of the request. I did it anyway because leaving the pair asymmetric would be surprising, happy to drop it if you would rather keep the change minimal.

The README's opening example is the case from the issue, two consecutive assertSeeIn with the same expected text, so I left it alone and documented the new form in the assertion list instead. Say the word if you want the headline example switched over too.

[kbond]

Thanks for the PR!

I've thought about this and I'm now thinking it could ambiguous what an array could mean: AND or OR?

I'd prefer to push users to custom browser methods: assertTitleIs('Post Title').

[Amoifr]

Thanks for taking the time to think it over, @kbond! Fair point: my own PR had to pick AND for assertSeeIn() and NONE for assertNotSeeIn(), which kind of proves there's no obvious answer. 😄 A custom browser method like assertTitleIs() says what it checks, so that's the better way. Thanks again for merging #213 and pushing it all the way to Playwright!