Licenses root

[nilshoerrmann]

While testing out #6738 and custom licensing, I noticed that Kirby does not have a root for licenses: a common folder where plugins should add their license files. There is a root for license which is the only root that resolves to a file directly and not to its folder, though.

I dicussed plugin licenses with @bastianallgeier a while back and we are currently preparing the following structure for our plugins:

So having $kirby->root('licenses') (plural) should allow us to access the parent folder grouping all licenses, with us putting our own licenses in a subfolder.