Fix TailwindCSS Intellisense extension

[FluxCapacitor2]

• I have read the note above about PRs contributing or fixing extensions
• I have tried reaching out to the extension maintainers about publishing this extension to Open VSX (if not, please create an issue in the extension's repo using this template).
  • Not my issue, but see Open VSX tailwindlabs/tailwindcss-intellisense#219 (comment)
• This extension has an OSI-approved OSS license (we don't accept proprietary extensions in this repository)

Description

This PR updates the TailwindCSS Intellisense (GitHub repo) extension entry in extensions.json to fix automated publishing to Open VSX.

Automated publishing stopped working after version 0.10.2, so I suspect the problem was initially caused by this commit.

This includes two changes:

1. I changed the extension package's root directory to packages/vscode-tailwindcss. This is where the source for the VS Code extension lives.
2. I switched from npm to pnpm. This is required because, as of this commit, the repository relies on the pnpm-specific workspace feature.

This works on my machine when testing with this command:

EXTENSIONS=bradlc.vscode-tailwindcss node publish-extensions.js

This generates an artifact at /tmp/artifacts/bradlc.vscode-tailwindcss.vsix, which I was able to install and use in VSCodium.

I looked at the GitHub Actions workflow file, and I couldn't see a step that installed pnpm, so I'm using it here via npx. The -y option disables prompting the user before automatically installing the package if it isn't found locally.