feat: add support for processing instructions

[Delta-official]

Adds support for processing instructions (<?target data?>). This probably needs a major bump as the Emitter trait got new methods on it and the enums for events got a new variant.

Sadly the html5lib-tests tests don't pass (at least not yet) because they rely on the old behaviour of interpreting every tag starting with a <? as a bogus comment. Not really sure what to do about this; I might try upstreaming the required changes.

Closes #144

[untitaker]

i think there have been some changes to html5lib-tests where these tests are now maintained in another repo. you'd have to search the issue tracker for it. if so, i would really like to switch to that repo

[Delta-official]

From the looks of it, the tokenizer tests never ended up getting moved to wpt, so I'm unsure what to do

[untitaker]

i've caught up with the changes in html5lib-tests and it seems that tokenizer tests are still "supposed to be" maintained. so i think the right course of action is to file a PR upstream, then update the git submodule here to use these new tests.

this will then break the tree builder tests in html5gum. i'm fine with either having a separate html5lib-tests checkout for those, or if you want to take on the wpt migration that would be greatly apprechiated too (i haven't checked how much work that would be)

[Delta-official]

Tree builder tests should be compatible with the ones in html5lib-tests (as they use similar, if not identical, formats), so I think I'll try to do the migration for tree building tests

[Delta-official]

Funnily enough, I'm blocked by html5ever's lack of processing instruction support, so I'll have to fix that there first to migrate the tree building tests :)