[2.x] Add support for line decorations

[ryangjchandler]

You can now apply decorations to a specific line, which right now means adding classes to a line of your choosing.

(new Phiki)
    ->codeToHtml(...)
    ->decoration(new LineDecoration(0, new ClassList(['my-class']))

The first line (0-indexed lines) will now have the my-class class added to it.

I would like to add support for Shiki's character decorations too in the future, but there's quite a bit of logic involved with that since they can split tokens into multiple smaller tokens, creating wrapper elements, etc.