Add type annotations

[DivineDominion]

This PR adds type annotations esp. to the Document type so that in apps, tools have an easier time figuring out that $document->getYAML() ?? [] is valid.

At least in some cases -- I was surprised that string is also possible! So good to know, need to work around that.

• Honestly, I'm not sure how to deal with this w.r.t. releasing new versions of the package and making sure that the minimum PHP version is installed to support these annotations :)
• The test case function return type annotations of void were added to comply with tools to check consistency (phpstan) mostly; I don't feel strongly about adding or keeeping them out either way. Can revert that commit if you want.

[mnapoli]

Tests are failing, we could remove support for PHP 7.4 if that helps?

[DivineDominion]

Ugh, of course I just ran the tests locally and didn't think of CI and older PHP's!

@mnapoli What's a sensibel schedule to drop support for older language versions in the PHP community? I can't help, I'm new :)

[mnapoli]

7.4 is ancient, no problem at all to remove support from it. 8.0 is possible too.

[DivineDominion]

@mnapoli Could you rerun tests with PHP 8 to check? I tested with PHP 8.2 and 8.3

[mnapoli]

Even though the build was canceled it seems the tests passed:

[DivineDominion]

Yeah looks like an odd timeout?

WDYT about a php-8 branch, then I change the merge target to that? Dropping PHP 7.x support, one can probably make other changes while at it

[mnapoli]

This wasn't a timeout, this is normal the job was cancelled because teh 7.4 job failed.

So we just need to remove 7.4. Can be in another PR or here.