[10.x] Add public path to application contract

[jnoordsij]

Requires #45968 to be merged first.

This MR adds the publicPath function the the Application contract for consistency with other path-related helpers. It also updates the global helper function to use this function.

[jnoordsij]

Tests suggest I've hit a weird spot here: path.public can be overridden as singleton in the Applications class, which does (before this MR) change the behavior of the public_path helper but not the publicPath function; this seems somewhat inconsistent. Moreover, there's various other paths that can be set, with some of them ignored in both the helper and Application function (e.g. path.config) and some of them being used in both places (e.g. path.storage).

Looking for a bit of input here, but I think the best take would be to create a publicPath property and setter, similar to those for config, and use those consistently.

[jnoordsij]

I've limited this MR to just adding the method to the contract and moving the function definition to inside the Application class; working on consistency of path overriding in https://github.com/jnoordsij/laravel-framework/tree/rework-application-public-path-helper.