This commit enhances the `CompileFn`, which is used to process each
entry-point, to support running a passed-in callback (and wait for it to
complete) before proceeding with writing the transformed files to disk.
This functionality is currently not used. In a subsequent commit, it
will be used for passing info from worker processes to the master
process that will allow ngcc to recover when a worker process crashes in
the middle of processing a task.
PR Close#36626
Previously, the "Compiling <entryPoint>" log message was printed before
starting to analyze and transform files, but after creating the
`EntryPointBundle` (which includes creating the TS program).
Since creating the `EntryPointBundle` involves some work, it is more
accurate to move the log message before creating the bundle.
PR Close#36626
