[9.x] Add destroyable singletons

[axlon]

This PR aims to create an easy way to mark singleton routes as 'destroyable'. A destroyable singleton may be deleted, but it may not be created (by default). This is useful when its a resource with a default state and deleting it just resets it to that state, or when it is a resource the user cannot create on his own.

Before:

Route::singleton(...)->creatable()->except('create', 'store');

After:

Route::singleton(...)->destroyable();

When singleton routes came out we rewrote our routes to make use of them, but 8.42.2 (6ddf3b0) changed how they worked by not making them not have a destroy action by default anymore, which this PR is trying to solve.