If a webapi has a []const u8 parameter, then the page.call_arena is used. This
is our favorite arena to use, but if the string value has a lifetime beyond the
call, it then needs to be duped again (using page.arena).
When a webapi has a Env.String parameter, the page.arena will be used directly
to get the value from V8, removing the need for an intermediary dupe in the
call_arena.
This allows HTMLCollections to be streamlined. They no longer need to dupe the
filter (tag name, class name, ...), which means they can no longer fail. It also
means that we no longer need to needlessly dupe the string literals.
