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

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#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` ](https://github.com/facebook/docusaurus/blob/ae5328daac080418c485ac9ee3393a4270d431e4/packages/docusaurus-plugin-content-docs/src/versions/files.ts#L53). files.

It doesn't mention this in the documentation on [versioning](https://docusaurus.io/docs/versioning#creating-new-docs) - 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.