Make the cache inside each Virtual Node consistent with the global cache API

In conjunction with the change to [allow setting a default value through the global `cache.get`](https://github.com/dequelabs/axe-core/issues/3276), it would be great if the [individual cache inside each Virtual Node](https://github.com/dequelabs/axe-core/blob/develop/lib/core/base/virtual-node/virtual-node.js#L29) also had the same API. Currently we have to do a lot of [`if (!this._cache.hasOwnProperty(value))` checking](https://github.com/dequelabs/axe-core/blob/develop/lib/core/base/virtual-node/virtual-node.js#L115) to determine if we need to set an item in node cache. It would be a lot more convenient if we could use a `get` with a default value API.

One way to achieve this is if the cache file had a function to instantiate a new cache with all the APIs, and each Virtual Node called this function to create the individual cache. The global axe cache could also use this function to setup the `cache` export.