YouTube video are displayed inline too

[laurent22]

When markdown.plugin.externalEmbed is enabled, it should only display YouTube videos when the URL is on its own line.

However currently it also display them when the URL is inline.

[Shresthap21]

@laurent22 I've submitted a PR to fix this: #14356

The solution checks for surrounding text content, YouTube URLs are only embedded when they appear standalone (no text before/after in the same paragraph). Inline URLs now render as regular links.