build: replace node/npm with tailwindcss cli (v3.4.4)

[cjshearer]

This is a test for replacing node/npm with the standalone tailwindcss cli, using the new css.Tailwind feature introduced in hugo 0.128.0. Since css.Tailwind feature is intended for tailwind v4, which does not yet have a --config option, I cannot directly use module mounts to mount a tailwind.config.js file provided by a theme/module. However, this PR contains a workaround where we use the HUGO_FILE_TAILWIND_CONFIG_JS provided by hugo to find the theme's tailwind.config.js. By having the exampleSite (or other calling module) use a different name for the theme (e.g. tailwind.config.mjs), which is still recognized by tailwindcss, but is not recognized by hugo, the calling site can use a thin wrapper which calls out to the theme's tailwind config.

An interesting side-effect of this is that consumers of this theme will be able to override/extend the tailwind configuration without vendoring, which is actually pretty cool!

[cjshearer]

One consideration with this method is that installing multiple themes, each with different tailwind.config.js files, is almost certain to break. Only the first tailwind config that gets resolved will be supplied, which makes this very brittle.

If I could refactor my tailwindcss stuff to be compatible with the v4 alpha (in other words, implement a subset of tailwindcss/typography within main.css) then I wouldn't need a config, I could use v4, and I could import multiple (tailwind v4 compatible) themes.