Allow new path placeholders to be registered

[Joachim Van Herwegen]

Feature description

When using file paths, @css: can be used to reference paths relative to the CSS root folder, as defined here:

CommunitySolidServer/src/util/PathUtil.ts

Line 286 in 486241f

 export const modulePathPlaceholder = '@css:'; 

It could be useful to allow other placeholders to be registered. This way external components can make use of the same path utilities.