fix(v2): move metadata export after compiling MDX to avoid weird MDX parsing error.

[endiliey]

Motivation

Move metadata export after compiling the MDX to avoid weird MDX parsing error.
Although we've fixed #2095 (MDX thinking that metadata was part of paragraph), there might be some other unexpected case.

For example:

This will error out, because it is unclosed (funnily it thinks its unclosed because style must always have </style> counterpart. And MDX think metadata is part of the JSX

## Hello World
<style dangerouslySetInnerHTML={{__html: `
  .comparisonTable td {
    width: 25%;
  }
  .comparisonTable .middle {
    text-align: center;
    vertical-align: middle;
  }
  .comparisonTable ul {
    padding-left: 1em;
  }
`}}/>

export const metadata = {
  id: 'test'
}

https://mdxjs.com/playground

If its closed properly, it doesnt error. Now I'm pretty confused how they determine HTML or JSX

## Hello World
<style dangerouslySetInnerHTML={{__html: `
  .comparisonTable td {
    width: 25%;
  }
  .comparisonTable .middle {
    text-align: center;
    vertical-align: middle;
  }
  .comparisonTable ul {
    padding-left: 1em;
  }
`}}></style>

export const metadata = {
  id: 'test'
}

A real scenario is https://github.com/CanopyTax/single-spa.js.org/blob/master/docs/separating-applications.md

Now, we will just do the export on top level on mdx-loader part. See code in this PR packages/docusaurus-mdx-loader/src/index.js

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

Test with this markdown. No more error.

Hot reload still working and everything

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[yangshun]

Will all of our markdown/MDX files have front matter? I think it's a requirement for us but not part of the spec. Wonder if this breaks for users who have markdown files which don't have front matter.

[endiliey]

No it doesnt need to have frontmatter. Actually Ive always tested docusaurus 2 building 1000 files without any frontmatter at all. It also wont break if it doesnt have frontmatter. You can give it a try

Btw the frontMatter export code was written by you 😂

[endiliey]

frontMatter export PR here #1450

Authored by @yangshun

[yangshun]

Ooops. Obviously my memory is quite bad 😅

…

On Tue, Dec 10, 2019 at 8:14 PM Endi ***@***.***> wrote: frontMatter export PR here #1450 <#1450> Authored by @yangshun <https://github.com/yangshun> — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#2105?email_source=notifications&email_token=AAKBCHI35RJUXVJAT7HK6PDQYBSJ3A5CNFSM4JYIIXZ2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEGR2FPY#issuecomment-564372159>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAKBCHKM4FHJIBYCI4LK67LQYBSJ3ANCNFSM4JYIIXZQ> .