Add a clear all button to recent searches

Lets users wipe their whole search history at once from the recent
searches section header, as per the designs.

UnitTests/Sources/SearchScreenViewModelTests.swift

@@ -162,6 +162,15 @@ struct SearchScreenViewModelTests {
         #expect(userSettings.searchBreadcrumbs.isEmpty)
     }
     
+    @Test
+    func clearingBreadcrumbs() {
+        userSettings.searchBreadcrumbs = [.query("Second"), .room(roomID: "2")]
+        
+        context.send(viewAction: .clearBreadcrumbs)
+        
+        #expect(userSettings.searchBreadcrumbs.isEmpty)
+    }
+    
     @Test
     func breadcrumbsAreDisplayed() async throws {
         staticRoomListSubject.send([.mock(id: "2", name: "Second")])