Use Tailwind CSS standalone CLI instead of NPM

[justinmayer]

Adds pytailwindcss Python package as a dependency, removing need for Node.js / NPM to be installed.

Other notes:

• This PR is currently focused on Tailwind CSS v3 and will probably not function correctly on v4.
• Only Tailwind CSS plugins that are bundled with Tailwind CSS standalone CLI v3 are supported — adding Tailwind CSS plugins to the TAILWIND Pelican setting is no longer supported.
• If someone wants to submit a follow-up PR to re-instate the removed Node.js/NPM-based functionality via a configuration setting along the lines of what I mentioned previously, that would be welcome.
• v0.3.0 of this plugin fixed one problem but may have created another — changing the working directory may have caused Tailwind to no longer find templates, resulting in an output.css file that is missing needed utility classes.
• v0.2.0 of this plugin does not appear to function correctly after the release of Tailwind CSS v4, so at present the only version of this plugin that seems to work correctly is the code in this branch, and only then if you set "TAILWIND": {"version": "3.0.24"} in your Pelican settings file (pelicanconf.py).
• Follow-up pull requests from volunteers to add support Tailwind CSS v4, including corresponding additions to the test suite, would be welcome and much appreciated.

Closes #4