Refactor + add more tests (Part 1)

[endiliey]

Motivation

1. We have lot of duplicate codes especially in server.js and generate.js
2. We need to add more tests to catch regression
3. Refactoring make it easier to migrate to Docusaurus v2

Changes

1. Refactor duplicate mdToHtml function in server.js and generate.js
   https://github.com/facebook/Docusaurus/blob/a7a214fb3a36b3627e4346c2ddbcc4d01e3bf3a1/lib/server/generate.js#L108-L122
   https://github.com/facebook/Docusaurus/blob/a7a214fb3a36b3627e4346c2ddbcc4d01e3bf3a1/lib/server/server.js#L198-L212

2. Move out core/routing.js to server/routing.js because it is more related to Express.js routing

3. Refactor & Add tests for getLanguage and getSubDir in server/utils.js 😄

4. Refactor duplicate isSeparateCss function in server.js and generate.js
   https://github.com/facebook/Docusaurus/blob/a7a214fb3a36b3627e4346c2ddbcc4d01e3bf3a1/lib/server/server.js#L118-L128
   https://github.com/facebook/Docusaurus/blob/a7a214fb3a36b3627e4346c2ddbcc4d01e3bf3a1/lib/server/generate.js#L74-L84

5. Refactor duplicate insertTableOfContents function in server.js and generate.js to core/toc.js & add test for it
   https://github.com/facebook/Docusaurus/blob/a7a214fb3a36b3627e4346c2ddbcc4d01e3bf3a1/lib/server/generate.js#L60-L70
   https://github.com/facebook/Docusaurus/blob/a7a214fb3a36b3627e4346c2ddbcc4d01e3bf3a1/lib/server/server.js#L106-L116

6. Moved core/getTOC.js function to core/toc.js so now core/toc.js has two functions:

• getTOC
• insertTOC

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

1. Test Locally

yarn start

2. Test in netlify
   https://deploy-preview-847--docusaurus-preview.netlify.com/

[docusaurus-bot]

Deploy preview for docusaurus-preview ready!

Built with commit 3ad88a7

https://deploy-preview-847--docusaurus-preview.netlify.com

[yangshun]

This is awesome @endiliey! Tests are great