add local video support

[cloverich]

• support drag and dropping local video files. Video files are uploaded then displayed inline
• parse and serialize video elements in markdown using image syntax

Video files are stored in markdown as images: ![my cool video](my-cool-video.mov). When parsing (markdown -> mdast -> slate), the markdown parser checks image tags for extensions, and when found, creates a video Slate node instead of an image node:

{
  type: 'video'
  url: 'chronicles://my-cool-video.mov'
  // ...
}

To render, the added VideoElement is included into the Plate components list; the implementation is naive. (On second pass, I added Captioning and media toolbar for delete).

Closes #54

[cloverich]

Worth comparing to the upload = async (dataUrl: string): Promise<UploadResponse> implementation above; I had to implement the above one like that because Plate's createImagePlugin reads the dropped file as a dataUrl before handing off.

By comparison, (as I learned) you can directly hand off an ArrayBuffer (I believe) from the dropped file and stream it to the filesystem. This saves the need to read the file into memory again as a dataUrl, and then to encode it from dataUrl back to binary. Probably not super noticeable performance wise, but its notable because the implementation is actually simpler in addition to being more performant; I think generic file handling could probably merge all the plugins together and go through this method.

[cloverich]

Notably, plates createImagePlugin doesn't do this, I'm not sure the reason. But the effect is, if this plugin comes before it, this routine will never be called, and dropping a video file will silently fail.

[cloverich]

Will do:

• Remove MediaEmbed (unused, thought it was already here)
• Comment on slate node structure
• Re-name slateInternal
• Add caption / delete dropdown to video (if its free, like images)