refactor(mdx-loader): streamline typescript usage for remark plugin types

[lebalz]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

I wrote a remark plugin and was not sure how i have to type the plugins correctly. So i checked with docusaurus' default plugins and saw that they are not properly typed aswell and that the structure between the remark-plugins is not really streamlined.

In my research i found the solution: type the plugins with Plugin<PluginOptions[], Root> and let them return a Transformer<Root> (instead of Transformer only). Like that, TS can infer the types directly:

const plugin: Plugin<PluginOptions[], Root> = function plugin(
  options,
): Transformer<Root> {
  return async (root, vfile) => {
    const {visit} = await import('unist-util-visit');
    visit(root, 'link', (node, index, parent) => {
      // link is now known to be of type Link

      // parent is of type `Parent | undefined`  and index: `number | undefined`
      // before, parent was of type `never`
      if (index === undefined || !parent) {
        return;
      }
    }
  }
}

A helpful resource was: 👉 https://unifiedjs.com/learn/recipe/narrow-node-typescript/

Test Plan

All tests still pass

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[slorber]

awesome 👍 thanks for the cleanup!