Add support for versioned sidebars which are `.js` not `.json`

[robert-j-webb]

Have you read the Contributing Guidelines on issues?

• [X] I have read the Contributing Guidelines on issues.

Prerequisites

• [X] I'm using the latest version of Docusaurus.
• [X] I have tried the npm run clear or yarn clear command.
• [X] I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• [X] I have tried creating a repro with https://new.docusaurus.io.
• [X] I have read the console error message carefully (if applicable).

Description

Version sidebars are hard coded to be .json . files.

It doesn't mention this in the documentation on versioning - it just kind of implies it.

Maybe there should be an alert warning people about it? or there could be some kind of fix that would make it possible to use either.

Here's my proposed fix:

Make the getVersionSidebarsPath function read the user's source code to see if it's .js or .json before it returns. I'm sure a more elegant way exists but this seems fine, unless I'm missing something.

Reproducible demo

https://codesandbox.io/p/devbox/priceless-goldwasser-qdeo7?file=%2FREADME.md&privacy=public

Steps to reproduce

1. Make a new version `npm run docusaurus docs:version 1.1.0

2. Replace the version-1.1.0-sidebar.json with an equivalent .js file like:

const sidebar = {
 tutorialSidebar: [
   {
     type: "autogenerated",
     dirName: ".",
   },
 ],
};

module.exports = sidebar;

3. Observe a crash

Expected behavior

Versioned sidebars should behave like normal ones, allowing js (or even ts)

Actual behavior

It crashes, it can't find the sidebar

Your environment

• Public source code:
• Public site URL:
• Docusaurus version used:
• Environment name and version (e.g. Chrome 89, Node.js 16.4):
• Operating system and version (e.g. Ubuntu 20.04.2 LTS):

Self-service

• [X] I'd be willing to fix this bug myself.