[Flight] Allow aborting during render (#29764)

Stacked on #29491

Previously if you aborted during a render the currently rendering task
would itself be aborted which will cause the entire model to be replaced
by the aborted error rather than just the slot currently being rendered.

This change updates the abort logic to mark currently rendering tasks as
aborted but allowing the current render to emit a partially serialized
model with an error reference in place of the current model.

The intent is to support aborting from rendering synchronously, in
microtasks (after an await or in a .then) and in lazy initializers. We
don't specifically support aborting from things like proxies that might
be triggered during serialization of props

Author: gnoff