This is to pave the way for introducing a new "Page" container, which will take
over the page lifecycle currently burdening Session. The ultimate goal of that
is to allow the Session to have multiple pages (mostly for better transitions
between pages), which is hard to do now since the Session has so much state.
This rename was aggressive, e.g. currentPage() -> currentFrame() so that, when
the new Page container is added, you won't see "currentPage()" and wonder:
"Does 'currentPage' mean the new Page container, or the Frame (which
used to be called Page)".
@import("lightpanda") where needed.
Would also like to do this for String, Page, Session and js which all stand out
as types that are use across the codebase.
I know that a few devs are doing this in new work and I haven't heard anyone
voice an objection.
