Enable generating stubs in parallel (multiprocessing)

[lagru]

Release note

For maintainers and optionally contributors, please refer to changelist's README on how to document this PR for the release notes.

Add the new experimental `--workers` option to `docstub run` to generate stub files
in parallel. This can reduce runtime significantly for projects with many files.
For now, multiprocessing has to be explicitly enabled by requesting a number of
workers.
{label="enhancement"}

Ensure that docstub still returns a non-zero exit code when it is run with output
completely disabled (`-qq`). The same applies when running docstub with 
`--quiet --fail-on-warnings`.
{label="fix"}