Add support for embedding playlists

[gfscott]

Closes #149

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

1. Embed a playlist that starts playing from the beginning. These URLs only include a Playlist ID and therefore don't work in Lite mode:
   https://www.youtube.com/playlist?list=PLFtSvldL7Mh4ismj4BgH33pBR9hbtBkxz
2. Embed a single video as part of a larger playlist. These URLs include both a Video and a Playlist ID, and often the video's index, or position within the playlist. In Lite mode, these URLs will render the single video, instead of the playlist.
   https://www.youtube.com/watch?v=C04JZsoqs1A&list=PLFtSvldL7Mh4ismj4BgH33pBR9hbtBkxz&index=4

Known limitations in Lite mode

lite-youtube-embed doesn't recognize playlist IDs so if there's no video ID, the embed won't display anything in Lite mode. But if it finds a video ID (such as in the second example above) it will embed just that video.

[changeset-bot]

🦋 Changeset detected

Latest commit: d8dbccb

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 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);

Typo in the property name 'responive' should be 'responsive'.

Suggested change

	t.is(embed(extract(testString), override({lite: { responive: true }}), 1),
	t.is(embed(extract(testString), override({lite: { responsive: true }}), 1),