chore: Explicitly use Prettier in VS Code for MDX

[samestep]

I haven't yet noticed any discrepancies between Prettier and what VS Code is currently doing, but this can't hurt.

This is a followup to #899: https://github.com/silvenon/vscode-mdx/blob/v0.1.0/package.json#L26

[codecov]

Codecov Report

Merging #905 (7b48849) into main (27a99ee) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #905   +/-   ##
=======================================
  Coverage   68.17%   68.17%           
=======================================
  Files          62       62           
  Lines        8117     8117           
  Branches     1768     1768           
=======================================
  Hits         5534     5534           
  Misses       2576     2576           
  Partials        7        7           

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