Tab manager update: Add a feature flag (#6372)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/72649045549333/task/1210728360131191?focus=true

### Description

This PR adds a new feature flag and caches its value in the
`TabSwitcherViewModel`.

### Steps to test this PR

QA-optional


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  - https://app.asana.com/0/0/1210760591708462

Author: 0nko

app/src/main/java/com/duckduckgo/app/tabs/TabFeatureFlags.kt

@@ -36,5 +36,7 @@ interface TabManagerFeatureFlags {
     @Toggle.DefaultValue(DefaultFeatureValue.TRUE)
     fun tabInsertionFixes(): Toggle
 
-    // There will be a new feature for the new design of tab manager
+    @Toggle.InternalAlwaysEnabled
+    @Toggle.DefaultValue(DefaultFeatureValue.FALSE)
+    fun newToolbarFeature(): Toggle
 }

app/src/main/java/com/duckduckgo/app/tabs/ui/TabSwitcherViewModel.kt

@@ -166,6 +166,10 @@ class TabSwitcherViewModel @Inject constructor(
     private val selectionMode: Selection
         get() = requireNotNull(selectionViewState.value.mode as Selection)
 
+    val isNewDesignEnabled: Boolean by lazy {
+        tabManagerFeatureFlags.newToolbarFeature().isEnabled()
+    }
+
     sealed class Command {
         data object Close : Command()
         data class CloseAndShowUndoMessage(val deletedTabIds: List<String>) : Command()