RenderPlugin: support relative paths

Is your feature request related to a problem? Please describe.

When using the RenderPlugin you have to specifiy paths relative to the project dir. This is very annoying when you want to put the template and the related css/js file in the same folder.

Describe the solution you'd like

At least for .liquid templates pass tagToken.file (the file where the tag is in) as context to shortcodeFn (https://github.com/11ty/eleventy/blob/main/src/Engines/Liquid.js#L172). This would allow the RenderPlugin to access file from context. When file is available the RenderPlugin could first try a relative path based on path.dirname(file) then fallback to a project relative path and finally error out if path is not found

Describe alternatives you've considered

No response

Additional context

No response