Add support for webm format in Picture component video check

[goekce]

Fixes: jupyter-book/mystmd#2996

Related mystmd component PR: jupyter-book/mystmd#2999

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Latest commit: 415b52f

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[Copilot]

Pull request overview

Adds .webm detection to the Picture component so WebM sources are treated as videos (matching existing .mp4 behavior) in the React renderer.

Changes:

• Extend the video-source check in Picture to include .webm.
• Adjust the if condition layout around the video check (introducing a formatting inconsistency that should be cleaned up).

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