ENH: Allow dict for times in plot_evoked_joint(#13289)

[shruti423]

This PR enhances the times parameter in plot_evoked_joint (and effectively plot_topomap) to accept a dictionary, addressing issue #13289.

Previously, times="peaks" hardcoded the search to 3 peaks over the full epoch. This update allows users to specify:

1. Exact peak count: times={"peaks": 5} (finds 5 peaks over the whole epoch).
2. Specific time windows: times={"peaks": [(0.0, 0.1), (0.3, 0.5)]} (finds one peak per window).

I modified mne/viz/utils.py to handle the dictionary logic inside _process_times, ensuring internal consistency across both plotting functions as suggested by @drammock.

Checklist

• Documentation updated (updated docstring in evoked.py with new usage and np.linspace hint).
• New tests added (added test_plot_joint_times_dict in test_evoked.py).
• Existing tests passed (ran test_evoked.py, and utils.py locally).

Related Issue

Fixes #13289