chore(gatsby-plugin-mdx): refactor mdx-loader async code

[pvdz]

This function was not optimally using the syntactic sugar of async functions. It was manually creating and returning a promise, including dealing with exceptions. Instead, the async keyword takes care of all of that for you implicitly. So this simplifies the function a little bit.

Suggest to review without whitespace.

[laurieontech]

Should this merge into master or merge into #25757?

[pvdz]

#25757 is likely to merge to master long after this one so I'll rebase that PR to master once it does and resolve the inevitable merge conflict.

[pvdz]

Once I fix the tests ... 🤔

[gatsby-cloud-staging]

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[pvdz]

I'm going to need more time to validate this is not going to break webpack so I'm going to put it in draft mode and return to it after my vacation,

[wardpeet]

If you're not sure you can always do this:

module.exports = function loader(content) {
  const callback = this.async();

  mdxLoader
      .call(this, content)
      .then(args => callback(null, args), err => callback(err));
}

async function mdxLoader(content) {