feat(v2): add ability default lang for code blocks

[lex111]

Motivation

It often happens that in Markdown code blocks do not (intentionally) specify the language, and then the default language was used. This is convenient if the examples language in docs is the same (why should it be indicated all the time?). Or, for example, when end-user forgot to specify that language.

Note: if you do not specify a language, then in HTML it will be represented as language-undefined.

In v1 there was such an feature, I propose to bring back it to v2.

Also in this PR, as an experiment, changed way to provide options for Prism - in a separate object (as it was in v1). This is a breaking change, yes, but in the end it is a flexible approach, because in the future there may be more options for highlighting code (=Prism).

- themeConfig.prismTheme
- themeConfig.prismDefaultLanguage
+ themeConfig.prism: {
+   theme: ...
+   defaultLanguage: ...
+   // and so on
+ }

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

1. Set the following options in docusaurus.config.js:

// ...  
  themeConfig: {
  // ...
    prism: {
      defaultLanguage: 'js',
      theme: require(...),
    },

2. In docs, omit the language in the code block:

```
console.log('JS is my default language!')
```

3. Make sure that the given code block is highlighted as js:

console.log('JS is my default language!')

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[yangshun]

I'm ok with this. But I think we should alert people of the breaking changes by throwing an error in __DEV__ if prismTheme is being declared.

Also we need CHANGELOG changes and docs changes in https://v2.docusaurus.io/docs/markdown-features#syntax-highlighting.

@endiliey WDYT?

[endiliey]

i thought this is still wip ?

[lex111]

It was a proof of concept, to show you, I will soon complete this PR.

But I think we should alert people of the breaking changes by throwing an error in DEV if prismTheme is being declared.

Fine, although I would merely decide note in change log, given that this is an alpha version

[endiliey]

This should be fine. Seeing lot of people requesting dark and light mode config for the prism theme too. prism.theme.dark prism.theme.light or just prism.theme

…

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On 3 Nov 2019, at 11:49 PM, Alexey Pyltsyn ***@***.***> wrote: It was a proof of concept, to show you, I will soon complete this PR. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or unsubscribe.

[lex111]

Hmm, I have a problem, I don’t know how to document the new defaultLanguage field, because in the current version of docs v2 there is no section about Prism ;/

[endiliey]

We can start documenting here. https://v2.docusaurus.io/docs/theme-classic

Under new CodeBlock section.

[lex111]

Ready for review! ✔️

[endiliey]

I'm gonna stop https://v2.docusaurus.io/ auto publishing on master push. We should only deploy the docs after new version is out

[yangshun]

LGTM. cc @endiliey for another approval

[endiliey]

feel free to merge.

Squash and merge. please ✌️

[lex111]

So exciting, merged!