Retrieving channel adjacencies for our standard montages

I'd like to be able to do something like the following:

# %%
import mne

montage_name = 'easycap-M1'

m = mne.channels.make_standard_montage(montage_name)
adj = m.get_adjacency()

i.e., if I'm using one of our built-in montages and we are shipping a matching adjacency matrix, I want to be able to retrieve this matrix without having to worry about ch_names etc in the way I have to when using the current approach:

# %%
import mne

m = mne.channels.make_standard_montage('easycap-M1')
adj, ch_names = m.get_adjacency('easycapM1')  # now what do I even do with those ch_names? match them up with those in `m`??

Alternatively, we could have support for make_standard_montage(..., return_adjacency=True) to return the matrix alongside the montage.

WDYT?

cc @mmagnuski @sappelhoff @cbrnr

Start by examining make_standard_montage and the existing montage get_adjacency entry points described in the issue. Trace how built-in montage names and adjacency matrices are currently associated, especially the ch_names handling. Done means a caller can retrieve the matching adjacency for a standard montage without manually reconciling channel names, with the chosen API behavior covered by tests.