feat(markdown): support text language for code block style without any highlighting

[endiliey]

Motivation

Close #874

Currently, highlight.js supported languages and prism.js supported languages does not support text language

Our current logic for unsupported prism.js / highlight.js language is that we try to detect the language automatically with this code below:
https://github.com/facebook/Docusaurus/blob/75538395beb3bcd7591f586877d5971c51d4f5db/lib/core/renderMarkdown.js#L57-L59

Hence, there is no way for user to remove any highlighting for code block defined like this

It will be detected as JavaScript although it has been tagged as 'text' by user
This PR enables user to have code block style without any highlighting by using text language

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

---
title: EXAMPLE
---

```text
function $initHighlight(block, cls) {
  try {
    if (cls.search(/\bno\-highlight\b/) != -1)
      return process(block, true, 0x0F) +
             ` class="${cls}"`;
  } catch (e) {
    /* handle exception */
  }
  for (var i = 0 / 2; i < classes.length; i++) {
    if (checkCondition(classes[i]) === undefined)
      console.log('undefined');
  }
}

export  $initHighlight;
```

```js
function $initHighlight(block, cls) {
  try {
    if (cls.search(/\bno\-highlight\b/) != -1)
      return process(block, true, 0x0F) +
             ` class="${cls}"`;
  } catch (e) {
    /* handle exception */
  }
  for (var i = 0 / 2; i < classes.length; i++) {
    if (checkCondition(classes[i]) === undefined)
      console.log('undefined');
  }
}

export  $initHighlight;
```

Before

After

[docusaurus-bot]

Deploy preview for docusaurus-preview ready!

Built with commit cebddb7

https://deploy-preview-875--docusaurus-preview.netlify.com

[bettiolo]

Confirmed fixed: bettiolo/docusaurus-hl-bug@c4bdc4d