split off simulation code into separate package

Hi @mne-tools/mne-python-steering-committee

@ctrltz and colleagues have been doing some work on simulating MNE objects (above and beyond what we currently support), in the new https://github.com/ctrltz/meegsim package. They have expressed interest in incorporating their work into MNE-Python, and after a few discussions with @larsoner @agramfort and @britta-wstnr we've settled on a tentative plan to "spin off" our existing simulation code into a new package (mne-simulation or so) similar to what was done for connectivity a few years ago, and then give @ctrltz and colleagues write-access to that repo so they can add their code and also have a much smaller/faster test/CI suite. Existing simulation code would be deprecated in MNE-Python release 1.9 and removed in 1.10.

This issue is your chance to weigh in on whether you think it best to:

1. follow the above plan
2. keep existing simulation code in MNE-Python, and add new code into the main repo
3. keep existing simulation code in MNE-Python, and don't add the new MEEGsim functionality
4. something else (?)

No files, tests, or entry points are named. Start by reviewing the existing simulation code and the proposed mne-simulation or meegsim package plan, then follow the discussion to determine whether the package split is approved. Done would require an agreed migration and deprecation plan, but this issue does not define implementation scope.