Add option to show a zero line in browser

[cbrnr]

This PR adds an option to show a zero line (toggleable with "0") to the Matplotlib backend. I've implemented this feature for the Qt backend in mne-tools/mne-qt-browser#423.

Short example:

import numpy as np

import mne

mne.viz.set_browser_backend("matplotlib")

sfreq = 200.0
n_channels = 20
t = np.arange(int(60 * sfreq)) / sfreq

rng = np.random.default_rng(0)
offsets = rng.uniform(-100e-6, 100e-6, size=n_channels)
data = rng.standard_normal((n_channels, t.size)) * 20e-6 + offsets[:, np.newaxis]

ch_names = [f"EEG {i + 1}" for i in range(n_channels)]
info = mne.create_info(ch_names, sfreq, "eeg")
raw = mne.io.RawArray(data, info)

raw.plot(show_zero_line=True, scalings=dict(eeg=100e-6), n_channels=5, block=True)

[cbrnr]

The docs error (AssertionError: Duplicates in PYPI_PACKAGES and installer packages: ['mne-kit-gui']) is very likely unrelated.

The Azure error is also unrelated (hitting 403: rate limit exceeded).

[larsoner]

Hmm somehow it's hard for me to parse this. I expected at first that hitting "d" would change the position of the line in the plot. I think calling it "true zero" made me think of "where zero actually is for the signal". So maybe

Suggested change

	initialized. The zero line marks the true zero of each channel's own
	displayed trace, independent of any DC removal or highpass filtering
	already applied to the data. Can be toggled after initialization by
	initialized. The zero line marks the centering point of each channel's
	**displayed trace**, independent of any DC removal or highpass filtering
	already applied to the data. Can be toggled after initialization by

I wonder if there is value in having a third mode where toggling DC removal changes where the "zero line" is displayed so that it's actually at where the zero level is for that channel (in other words, treating "DC removal" as more of a visual vertical translation rather than a subtraction of any values from the trace)