tpl/css: Make default loader resolution for CSS @import and url() always behave the...

tpl/css: Make default loader resolution for CSS @import and url() always behave the same

Before this commit, we did dynamic loader resolution for CSS bundling for resources resolved by Hugo while any fallback to ESBuild would fall back to a (potentially) empty loaders config.

This revises the logic to always use a static list (see below) if `loaders` is not set. This should be easier do document and less confusing for the end user.

````
".png", ".jpg", ".jpeg", ".gif", ".svg", ".webp", ".avif",
".woff", ".woff2", ".ttf", ".eot", ".otf"
````

Fixes #14619