Add a SwiftUI modifier for easily tracking navigation between views

[Jeehut]

Most flexible "core" solution for #184.

Note that I considered but decided against the following names:

• .navigationPathChanged(to:) while being consistent with the extension function on TelemetryDeck, it's very uncommon for SwiftUI modifiers to state something like "changed" when nothing in the view hierarchy actually changed.
• .onNavigationPathChange() would be more SwiftUI-like, but how to pass the path in a natural wording? Doesn't seem perfect either.
• .reportNavigation(path:) seemed a good, but I liked track better because it's more commonly associated with Analytics.
• .signalNavigation(path:) is consistent with the signal method, but while I find TelemetryDeck.signal clear thanks to the TelemetryDeck context, in a SwiftUI view it seems a bit unclear.

For these reasons, I decided for trackNavigation(path:) which is clear and short and consistent with SwiftUI-naming.

[winsmith]

Simply lovely!