Adds support for Julia to be started with `--threads=auto|N[,M]` where
`N` specifies the number of threads in the default threadpool and `M`,
if provided, specifies the number of threads in the new interactive
threadpool.

Adds an optional first parameter to `Threads.@spawn`:
`[:default|:interactive]`. If `:interactive` is specified, the task will
be run by thread(s) in the interactive threadpool only (if there is
one).

Co-authored-by: K Pamnany <[email protected]>