Add resolveResource UI helper to complement the resolvePage UI helper

Antora should provide a UI helper that resolves any resource in the content catalog from a UI template. The new resolveResource UI helper would work just like the resolvePage UI helper, except it would allow the family to be specified. This helper will make it easier to reference assets from the UI that are stored in the content catalog.

Here's an example of how this helper could be used:

{{{#with (resolveResource 'ROOT::image$logo.png')}}
<img src="{{{relativize ./pub.url}}}">
{{{/with}}}

Like the resolvePage helper, it will resolve the resource reference relative to the context of the current page, if necessary. That context can be overridden using helper params. For example:

{{{#with (resolveResource 'logo.png' component='ROOT' version=null module='ROOT' family='image'}}}
<img src="{{{relativize ./pub.url}}}">
{{{/with}}}

The helper should also support an additional parameter named publishable that filters the resolved resource by whether or not it is publishable (e.g., publishable=true). By default, any resource will be resolved.