Make internal PHP functionality private

[bakerkretzmar]

This PR removes some methods and makes a few others private in Ziggy's PHP API. These methods were either for internal use or provided no real functionality of their own.

Notable changes

• The filter() method on the Ziggy class is now 'fluent' and returns an instance of Ziggy instead of a collection of routes.

Other changes

• The nameKeyedRoutes() and resolveBindings() methods on the Ziggy class, which are for internal use, are now private.
• The generate() method on the CommandRouteGenerator is now private.
• The applyFilters() and group() methods on the Ziggy class are now private. This might seem like a significant change, but it isn't. Both of these methods returned an array of Ziggy-formatted route definitions, which isn't particularly useful on its own, but more importantly, they were both redundant—Ziggy already uses these methods internally to filter its routes, so calling group() or applyFilters() directly has the same effect as configuring or passing the same options before instantiating a Ziggy object. The easiest way to understand this is to look at the changes to the tests in this PR: all of the data returned by applyFilters() and group() was already available in the namedRoutes key of the array that Ziggy returns.

Cleanup

• Removed the appendRouteToList() and isListedAs() methods, and code referencing them. These were both unused since the Router Facade macro was removed in Remove Route facade macros #306.
• Removed the except() and only() methods that didn't return anything and were already being called automatically when retrieving Ziggy's config. This functionality was inlined into the applyFilters() method.

[jakebathman]

Good to merge 👍