[ENH] Spatio-temporal cluster permutation testing: Define minimum "nodes" to define a cluster

For spatio-temporal cluster permutation testing, it's important to consider how many "nodes" make up a cluster.

For example:

• In the time domain (considering 1 "channel"): Are two neighboring significant time points already a cluster? Or does it start with "3", "4", "N"?
• In the spatial domain: the same but for neighboring channels who have are significant at the same point in time

Currently I can only assume that in MNE-Python, any neighboring relationship (2 timepoints, or 2 channels) makes up a cluster.

It would be really nice to

1. document this
2. offer a parameter to customize this

In FieldTrip, this is done as so:

cfg.minnbchan        = 2;          % minimum number of neighborhood channels that is
                                   % required for a selected sample to be included
                                   % in the clustering algorithm (default=0).

https://www.fieldtriptoolbox.org/tutorial/cluster_permutation_timelock/

xref https://github.com/mne-tools/mne-python/issues/4859

No implementation file or test is named. Start by locating the current spatio-temporal cluster permutation entry points and reviewing the linked FieldTrip reference; the work is done when the existing behavior is documented and the minimum-node behavior is specified, implemented, and covered by tests.