perf(v2): unblock metadata processing when possible

[endiliey]

Motivation

Await pauses the execution of current async function and waits for the passed Promise's resolution, and then resumes the async function's execution

In metadata.ts, we can actually increase the performance a little bit by making the readFile as promise, then continue resuming the function execution, only when we then really need the data. we await the promise.

I created a simple benchmark to sort of describe the situation
https://runkit.com/endiliey/await-readfile-vs-readfile-as-promise/1.0.0

Old approach is something like

async function oldApproach(source) {
  const {id, title} = await fakeReadFile(source);
  const version = getVersion(source);

  // do something else
  
  return {
    id,
    title,
    version,
  }
}

Now it's like

async function newApproach(source) {
  const frontMatterPromise = fakeReadFile(source);
  const version = getVersion(source);

  // do something else
  
  const {id, title} = await frontMatterPromise;
  
  return {
    id,
    title,
    version,
  }
}

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

Docs plugin is covered by tests, so since it passes, regression is less likely

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Built with commit 1a8a5e6

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