Reply Block: Embed referenced post when possible

[obenland]

Fixes #1027.

In this first pass the Mastodon embed provider only gets registered when the Reply block gets rendered. I'm open to making it generally available but initially it didn't feel like functionality that this plugin should be responsible for globally.

There's also an action to register additional Fediverse oembed providers beyond Mastodon.social. Not sure if there are others we should or need to consider.

The current implementation supports embedding posts from all supported oembed providers. I wasn't sure whether it should be limited to Mastodon or whether it should be all—please let me know if you have an opinion on that.

Finally, this maintains the anchor, that IndieWeb seems to be looking for.

Proposed changes:

• Adds oEmbed support to the reply block, automatically embedding content from supported platforms.
• Registers Mastodon as an oEmbed provider for better integration with Mastodon posts.

Other information:

• Have you written new tests for your changes?

Testing instructions:

Testing Reply Block with Mastodon Posts:

1. Create a new post.
2. Add a Reply block.
3. Enter a Mastodon post URL (e.g., https://mastodon.social/@obenland/113561729606054851).
4. Verify that:
   • The post is embedded above the reply link.
   • The reply link is still visible and functional.
   • The URL is properly formatted in the link text.

Testing with Non-Embeddable URLs:

1. Create a new post.
2. Add a Reply block.
3. Enter a URL that doesn't support oEmbed.
4. Verify that:
   • Only the reply link is shown (no embed).
   • The link is properly formatted.

Testing with Other oEmbed Providers:

1. Create a new post.
2. Add a Reply block.
3. Try URLs from other oEmbed-enabled platforms.
4. Verify that embeds work as expected while maintaining the reply link functionality.