refactor: add convenience HighlightLines::from_state constructor

[alexpasmantier]

This PR adds a convenience method to easily be able to reinstantiate a HighlightLines object from a previous state the user might have cached.

[Enselic]

Personally I would prefer a test that demonstrates how to use this new function, that also serves as a regression test.

But I'm fine you merging this as-is Keith if you are confident this is needed and the right approach.

[keith-hall]

I'm confident that this is needed and the right approach. I also agree that a test would be nice 🙂

[alexpasmantier]

Thank you both for your input 🙏🏻

I added a syntect::easy::HighlightLines<'a>::state method, without which the first one wouldn't be very useful and an associated test.

Didn't add the test as an example to the method's documentation but can do so if you think it's a good idea.

Cheers

[keith-hall]

Nice, thanks for the test. Looks good to me

[Enselic]

Looks good to me too, thanks.

I'd like to ask trishume to allow CI to run without manual action, but that's a later thing.

[alexpasmantier]

Thanks for the quick feedback 🙏
So should we merge this?