Enable non-tempdir lite-dir

[bollwyvl]

Problem

The use of the temporary directory means that none of the well-known paths will be picked up. Re-enabling these will require a lot more configuration switches to achieve relatively simple effects (e.g. extra labextensions, wheels, markdown-visible assets, etc).

Also, once significantly-sized extensions, pyodide, etc. come into play, the rebuild of a lite application can become quite slow if the cache has to be repopulated each time.

Proposed Solution

If jupyterlite_dir is specified in conf.py and is non-None, use that as the cwd for the subprocess.check_call, and don't delete any of the intermediate files.

Additional context

• n/a

[martinRenou]

If jupyterlite_dir is specified in conf.py and is non-None, use that as the cwd for the subprocess.check_call, and don't delete any of the intermediate files.

And we should definitely allow more options to the config 👍🏽