Allow specifying manual keys for workers to reuse them

[jkrumbiegel]

I've just thought about a use case that will be pretty common, where one wants to render a bunch of related qmd files in a project, but all with the same environment. Each file would get its own worker assigned and so even if they all share the same env, each worker startup will incur its own startup delay. Maybe one could allow a key in the front matter like daemon_id: 123 or so, if this was set project wide then all files would run with a single worker. A difficulty is that certain front matter settings need to be the same so that the worker is not force restarted when they change (like the env or exeflags keys).