Add stylelint extension

[florian-sanders]

Fixes #1648

• This PR adds an extension to wrap stylelint LSP,
• Stylelint is used to lint CSS content in files that support it (CSS, SCSS, JS, etc.),
• The VSCode Stylelint Extension is fairly popular with more than 2,595,360 installs,
• The LSP server comes from the stylelint/vscode-stylelint project but there are few things to note:
  • the GitHub repository contains both the LSP server and the VSCode extension code,
  • the LSP server is not entirely decoupled from VSCode extension code but it does work on it's own,
  • neither the LSP Server or the VSCode are published as npm packages,
  • the source code needs to be built (TypeScript project) & bundled to be used,
• I had to fork the original repository so that I could:
  • build & bundle the source code using the povided build-bundle command (see source package.json for more info),
  • publish it as an npm package: https://www.npmjs.com/package/@florian-sanders/vscode-stylelint-prebundled.

About the fork & npm package

I don't mind maintaining the fork & npm package but I'd definitely be okay to make it less about me and more about Zed or the community.
I chose to publish the LSP package in the @florian-sanders scope on npm just because I wanted to make it clear that it was not officially published by the Stylelint team but I'm open to other options.

For what it's worth, I have created an issue on the source repository to ask:

• if it was okay to fork (license allows it but just in case),
• if they are still planning on decoupling the server & extension code,
• if they plan on publishing the server as a standalone package.

You can follow the discussions here: stylelint/vscode-stylelint#623
For now nobody has answered.

[Mouvedia]

@ota-meshi @ybiquitous