[Security] Remove markdown-it-html5-embed

- Add tests for images with audio and video URLs.
- Remove `markdown-it-html5-embed`, which is quite old, depends on an outdated version of `markdown-it`, and loads in all of `mime-db` to check for audio or video URLs.
- Replace `html5-embed` with `html5-media`, a plugin based on the `markdown-it-html5-media` plugin.
- Use `mime/lite` to lookup MIME types for audio and video, in the browser.

package.json

@@ -68,16 +68,13 @@
     "markdown-it-container": "~4.0.0",
     "markdown-it-emoji": "~3.0.0",
     "markdown-it-footnote": "~4.0.0",
-    "markdown-it-html5-embed": "~1.0.0",
     "markdown-it-imsize": "~2.0.1",
     "markdown-it-sub": "~2.0.0",
     "markdown-it-sup": "~2.0.0",
     "markdown-it-table-of-contents": "~0.6.0",
     "markdown-it-video": "~0.6.3",
+    "mime": "~3.0.0",
     "rehype": "~11.0.0",
     "rehype-react": "~6.2.1"
-  },
-  "overrides": {
-    "markdown-it": "~14.0.0"
   }
 }