Allow frame to send the message port

Towards #2065

Handle the new communication pattern from the host side. Allows the
frame to send a single `'port'` message with a read `MessageChannel` in
stead of the `{'ready': true}` message signalling that the host should
send a message port. This will remove one hop in the communication
pattern.

Retains handling of the current pattern and does not update the frame
side yet. There are multiple implementations of both the host and the
frame behavior. All host implementations will be updated to allow either
pattern before the frame implementations are updated.

Change from a `if/else` chain to a `switch`. Use pattern matching to
destructure the `'data'` field in the case of exceptions.

pkgs/test/CHANGELOG.md

@@ -3,6 +3,7 @@
 * Change "compiling <path>" to "loading <path>" message in all cases. Surface
   the "loading" messages in the situations where previously only "compiling"
   message would be used.
+* Support browser tests where the frame creates the message channel.
 
 ## 1.24.4