Add playlist track length setting

[scruffian]

What?

Extracted from #75060.

Adds a showTrackLength setting to the Playlist block so track lengths can be hidden in the tracklist.

Why?

This extracts the independent track length visibility change from the larger playlist waveform work so it can be reviewed separately.

How?

The PR adds a showTrackLength boolean attribute, exposes it in the Playlist settings panel, applies hidden-length classes in editor and saved markup, and adds CSS to hide .wp-block-playlist-track__length when disabled.

Testing Instructions

1. Open the editor and insert a Playlist block with audio tracks.
2. Open the Playlist block settings.
3. Toggle "Show track length in Tracklist" off and confirm track lengths are hidden.
4. Toggle it back on and confirm track lengths are shown again.

Testing Instructions for Keyboard

1. Use the keyboard to focus the Playlist block.
2. Open block settings and tab to "Show track length in Tracklist".
3. Toggle it with the keyboard and confirm the track length visibility changes.

Screenshots or screencast

Use of AI Tools

This PR was prepared with assistance from OpenAI Codex. I reviewed the diff and am responsible for the changes.

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[jeryj]

This follows existing conventions, and works as described. I tested both options (with and without time shown).

Hiding the time on the frontend works well too. Using display: none on the wrapper should remove the length from being announced by the screen reader as well.