Add support for embedding playlists

[gfscott]

Closes #149

At long last, this PR adds support for YouTube playlists. There are two use cases:

Standalone playlists

These are YouTube playlist URLs that don't specify an individual video ID:
https://www.youtube.com/playlist?list=PLFtSvldL7Mh4ismj4BgH33pBR9hbtBkxz

For standalone playlists, the plugin embeds the player and starts playing from the first video in the list.

⚠️ In lite mode, the plugin simply returns the original URL. Playlists aren't supported by the lite-youtube-embed package.

Videos with associated playlists

These are YouTube URLs that include both a video ID and a playlist ID:
https://www.youtube.com/watch?v=C04JZsoqs1A&list=PLFtSvldL7Mh4ismj4BgH33pBR9hbtBkxz

For this type of URL, the plugin embeds the player and starts playing from the specified video, wherever it is in the playlist numbering.

ℹ️ In lite mode, only the individual video will be embedded. Playlists aren't supported by the lite-youtube-embed package.

[changeset-bot]

🦋 Changeset detected

Latest commit: fcde9ca

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
eleventy-plugin-youtube-embed	Minor
eleventy-plugin-embed-everything-demo	Patch
eleventy-plugin-embed-everything	Patch

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[Copilot]

Copilot reviewed 5 out of 8 changed files in this pull request and generated 1 suggestion.

Files not reviewed (3)

• packages/youtube/test/embed.default.test.js: Evaluated as low risk
• packages/youtube/test/_urls.mjs: Evaluated as low risk
• demo/src/youtube.md: Evaluated as low risk

Comments skipped due to low confidence (2)

packages/youtube/lib/embed.js:87

• The use of 'id ?? playlist' should be reviewed to ensure it doesn't introduce unintended behavior when both are present or missing.

return `https://i.ytimg.com/${fileTypePath}/${id ?? playlist}/${fileName}`;

packages/youtube/lib/embed.js:181

• [nitpick] The error message should be reviewed for consistency with other error messages in the codebase.

console.info("Couldn’t fetch video title. Falling back to default...\n", error);