Add ability to customize TabBarView's separator color

ios/FluentUI.Demo/FluentUI.Demo/Demos/TabBarViewDemoController.swift

@@ -251,6 +251,10 @@ extension TabBarViewDemoController: DemoAppearanceDelegate {
                                lightHighContrast: GlobalTokens.sharedColor(.teal, .tint40),
                                dark: GlobalTokens.sharedColor(.pumpkin, .tint40),
                                darkHighContrast: GlobalTokens.sharedColor(.burgundy, .tint40))
+            },
+            .separatorColor: .uiColor {
+                return UIColor(light: GlobalTokens.sharedColor(.red, .shade10),
+                               dark: GlobalTokens.sharedColor(.red, .tint40))
             }
         ]
     }

ios/FluentUI/Tab Bar/TabBarTokenSet.swift

@@ -20,6 +20,9 @@ public class TabBarTokenSet: ControlTokenSet<TabBarTokenSet.Tokens> {
         /// Font info for the title label when in landscape view.
         case tabBarItemTitleLabelFontLandscape
 
+        /// Defines the color of the top separator.
+        case separatorColor
+
     }
 
     init() {
@@ -48,6 +51,9 @@ public class TabBarTokenSet: ControlTokenSet<TabBarTokenSet.Tokens> {
                     assertionFailure("TabBarItem tokens are placeholders and should not be read.")
                     return theme.typography(.body1)
                 }
+
+            case .separatorColor:
+                return .uiColor { theme.color(.stroke2) }
             }
         }
     }

ios/FluentUI/Tab Bar/TabBarView.swift

@@ -207,5 +207,6 @@ open class TabBarView: UIView, TokenizedControlInternal {
                                                     forToken: .titleLabelFontLandscape)
             }
         }
+        topBorderLine.tokenSet[.color] = tokenSet[.separatorColor]
     }
 }