fix: avoid removing double extension for non-preprocessor assets

[ElMassimo]

Description 📖

This pull request makes some changes to ensure CSS filenames such as jquery.tree.css are not converted to jquery.tree in the ViteRuby::DevServerProxy.

Background 📜

Historically, stylesheet_link_tag in Rails did not support passing extname to asset_path, causing it to always append .css to any provided filenames.

This is problematic in Vite, because the dev server can serve file.scss.

For that reason, the DevServerProxy needed to convert that file.scss.css to file.css.

Since it's very uncommon for CSS files to have two extensions, the naive approach of removing the extra .css hasn't been a problem in practice, until #300.

The Fix 🔨

Restricting this patch to files with a known CSS preprocessor extension. The regex is more verbose, but should cause less problems.

In addition, support for extname was added in Rails 7, so in the future it will be possible to stop using this patch in the proxy.