MDX support

[scarf005]

from: denoland/deno#20383

I'm interested in writing this plugin. It'd be really helpful if i could get a pointer on where to start.

[dsherret]

@scarf005 A contribution for this would be very welcome! I'm not sure what mdx entails, but most likely it should be within the markdown plugin and be turned on when the file type is .mdx. Then perhaps those sections could be formatted by calling out to the typescript plugin. I'm going to transfer this issue there.