Add support for MDX

[coderinblack08]

trafficstars

Shouldn't be too bad since MDX is basically just markdown :D

Wait can I add file extensions manually rather than waiting for this to go through?

[Jason3S]

You can turn it on yourself.

1. Open an MDX file.
2. F1 Enable Spell Checking Document Language

Or go to VS Code settings and enable the filetype.

[sebastienbarre]

Unfortunately this does not seem to ignore React components, which is the goal of MDX, i.e. use JSX (React) in your markdown content. So any component is spell checked :(