feat(runtime): expose canGoBack for firmware chrome

Firmware needs to know whether a back control would go anywhere, and the
history that answers it belongs to the runtime.

native/include/lua/runtime.h

@@ -70,6 +70,9 @@ class Runtime {
   void requestLaunch(const std::string& path, const std::string& arg, bool replace);
   void requestBack();
   bool hasPendingNavigation() const { return pending_.kind != Pending::None; }
+  // Whether sys.back() would return somewhere rather than land on the launcher, which is what
+  // firmware chrome needs to decide whether to offer a back control.
+  bool canGoBack() const { return !history_.empty(); }
   // Loads whatever was requested. False means the app failed to start or history ran out at the
   // launcher, in which case no app is running.
   bool applyPendingNavigation();