feat: generate theme.json during dev mode using in-memory CSS

[eavonius]

This PR adds support for generating theme.json during development.

• Generates theme.json during vite dev.
• Resolves source css file path relative to resources/css during dev and public/build during a build.
• Uses the in-memory compiled css file (using hashed filename like app-*.css) during a build.
• Strips off query "?direct" from resolved path to css file when run during dev.
• Graceful error handling and clear warnings if CSS or Tailwind config is missing.

Tested with:

• Sage theme integration
• Both dev and production builds
• File watching and full reloads on CSS changes

[eavonius]

A complication I ran into, is that as soon as build/assets/theme.json exists (once running vite dev with this PR), it appears that when this code runs in the admin_head hook in setup.php for the sage theme:

$dependencies = json_decode(Vite::content('editor.deps.json'));

It finds the build directory there - and thinks this is a prod build. This errors out since it can't find the editor deps.

I didn't realize this was a problem until I tried to hop into the admin side of the site once the "built" theme.json exists. So I'm not sure what to do next.

[eavonius]

I was able to get this to work. It would require users add a line to setup.php to check that the wordpress environment is anything other than "development" to work though (this is the approach I took at least):

add_filter('admin_head', function () {
    if (! get_current_screen()?->is_block_editor()) {
        return;
    }

    if (defined("WP_ENV") && WP_ENV !== 'development') {
        $dependencies = json_decode(Vite::content('editor.deps.json'));

        foreach ($dependencies as $dependency) {
            if (! wp_script_is($dependency)) {
                wp_enqueue_script($dependency);
            }
        }
    }

    echo Vite::withEntryPoints([
        'resources/js/editor.js',
    ])->toHtml();
});

[retlehs]

Thanks for putting this together and for working through the editor.deps.json edge case

We've been working on an alternative implementation that takes a different approach: no new dependencies, no hardcoded paths, debounced atomic writes, and startup generation via transformRequest instead of reading raw CSS from disk. More details: #36

Thanks again