Add an event listener for `pause` to `debugger.js` and the necessary
glue to access it from the `devtools` crate. This returns important
information to know where we are paused, such as the source location and
frame state.
Fix frame and object actor encoding into messages. Use information from
`debugger.js` to correctly fill the fields.
Add a new `frames` list to the thread actor and handle the `frames`
message.
Fix `getEnvironment` reply in the frame actor. It is out of form (has a
`type` field but it doesn't require a followup empty message, it already
counts as a reply), so we need to handle it specially.
Note: For now we are focusing on the protocol side of the debugger, and
this patch only shows where the pause would happen. Pausing Servo itself
will happen in a followup.
Testing: `mach test-devtools` and manual testing. No errors (apart from
#42006).
Part of: #36027
---------
Signed-off-by: eri <eri@igalia.com>
Co-authored-by: atbrakhi <atbrakhi@igalia.com>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>
