ENH: Add mask_label_params parameter to plot_topomap

[CarinaFo]

What does this implement/fix?

Adds a new mask_label_params parameter to plot_topomap, allowing users to control the style of channel name labels
for masked (e.g. statistically significant) sensors

Currently, masked and unmasked sensor labels are styled identical when show_names=True and mask is provided

Closes #13474
follow up on #13598 (credit to @TaniaRocks for the initial approach)

Example

evoked.plot_topomap(
    mask=mask,
    show_names=True,
    mask_label_params=dict(fontsize='medium', color='green', fontweight='bold')
)

ToDo

• write test for mask_label_params

[drammock]

LGTM pending the suggestions!