Add canvas detection

[JackYoustra]

See #302

[JackYoustra]

@denisenepraunig why are the tests failing?

[Zandor300]

/Users/runner/work/DeviceKit/DeviceKit/Tests/Tests.swift:32:27: error: value of type 'Device' has no member 'isCanvas'
    XCTAssertFalse(device.isCanvas)
                   ~~~~~~ ^~~~~~~~

@JackYoustra isCanvas isn't available on tvOS. If you move your method in the Device class to be included on tvOS (and watchOS possibly), it should fix it.

https://github.com/devicekit/DeviceKit/runs/4045134200?check_suite_focus=true#step:4:705