The last boolean property has become inaccurate with this PR.
In the new situation, if the "usePreInstalledDriverExtension" boolean is set to `true` *and* a custom extension is installed, then it's certain that the Driver extension is not installed (unless  `addExtensions` is called with a Safari driver).

I'm going to implement `setSafariDriverExtension(File extension)` method, which allows a user to override the driver extension with their custom one. A positive consequence it that the safari driver can now assured that the Safari Driver Extension is always installed (previously, when the boolean was set to true, the driver had to assume that the driver was installed, which may not be the case if you're running multiple Safari instances in a grid).