Route BroadcastChannel messages over IPC so matching channels can
receive them across WebContent and WebWorker processes, rather than only
within a single process.
Each channel now serializes its payload, sends it upward over IPC, and
receiving processes deliver it locally after matching by storage key and
channel name.
Previously `BroadcastChannelRepository` held strong references to
`BroadcastChannel` instances, which is removed only after `close()` on
corresponding `BroadcastChannel` was called, which might never happen.
This would have to be revisited once we will implement broadcast
channels messaging across different WebContent processes, but for now
using weak references in the repository saves us from leaking all
unclosed `BroadcastChannel`s.
The repository being in static storage is a bit of a hodgepodge, but in
line with how our current storage partitioning is done. We should
eventually move this, along with other across browsing context APIs to a
proper location at a later stage. But for now, this makes progress on
the meat of the BroadcastChannel API.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
