@Nullable
	default ResourceRef resolve(String resource) {
		return resolve(resource, false);
	}
Given a resource string either returns a `ResourceRef` that identifies that
resource or <code>null</code> if the resolver wasn't able to handle the
resource string.

The method should _only_ return <code>null</code> if it does not recognize
the resource string, thereby informing the caller it should probably try some
other resolver. In case the resolver _does_ recognize the resource string as
having a supported format but something goes wrong and no reference can be
created it's normal to throw an exception.

Calls to this method are always considered untrusted.

@param resource The resource string to resolve
@return A <code>ResourceRef</code> or <code>null</code>
src/main/java/dev/jbang/source/ResourceResolver.java