Watch for added/changed static files (aka non-JS, non-CSS files) in watch mode

[timriley]

Right now adding new static files (e.g. app/assets/images/some-image.jpg) requires the hanami assets watch process to be restarted. This is far from ideal. Watch mode should detect added/changed files in assets directory and process those as required.

The https://github.com/tinchoz49/esbuild-plugin-copy-watch plugin may provide some inspiration for us here.