ENH: Add channel names to plot_alignment (#13502)

[mohit-malpote]

#13502.

What does this implement/fix? This PR implements the ability to display channel names directly on the 3D alignment plot.

A new parameter, show_names, is added to :func:mne.viz.plot_alignment. When set to True, it renders the channel name as a text label next to the corresponding sensor location in the 3D scene.

The implementation uses renderer.text3d to place labels for all standard sensor types (MEG, EEG, ECoG, SEEG, DBS).

A unit test has been added to mne/viz/tests/test_3d.py to ensure that text actors are correctly added to the scene when the parameter is used.

Additional information The scale of the text labels is set to be small (0.003) and the color is gray to avoid dominating the visualization.

A corresponding entry has been added to the change log using towncrier.