feat: add ConfigPlugin to auto-load module config files

[calebdw]

Hello!

In our project, we created a base service provider which allows every module to have its own config file (here we place things like the morph_map for that module and other module-specific configs). With the new plugin system, I figured this would be great to PR upstream.

Example:

abstract class ModuleServiceProvider extends ServiceProvider
{
    #[Override]
    public function register(): void
    {
        if ($this->app instanceof CachesConfiguration && $this->app->configurationIsCached()) {
            return;
        }

        $modules = $this->app->make('config')->get('app-modules.modules_directory');
        $module  = $this->moduleName();
        $config  = $this->app->basePath("{$modules}/{$module}/config/{$module}.php");

        if (file_exists($config)) {
            $this->mergeConfigFrom($config, $module);
        }
    }

    protected function moduleName(): string
    {
        return Str::of(static::class)
            ->afterLast('\\')
            ->before('ServiceProvider')
            ->snake('-')
            ->lower()
            ->toString();
    }
}

This has 3 commits:

1. I had to add an .editorconfig to keep my editor (neovim btw 🙃) from trying to convert tabs to spaces---feel free to customize
2. I added a new OnRegister attribute that allows plugins to hook into register
3. I added the new ConfigPlugin

Thanks!