Refactor sidebar styling to remove duplicate code

Multiplatform/Shared/Sidebar/SidebarStyleModifier.swift

@@ -0,0 +1,34 @@
+//
+//  SidebarStyleModifier.swift
+//  NetNewsWire
+//
+//  Created by Maurice Parker on 7/6/20.
+//  Copyright © 2020 Ranchero Software. All rights reserved.
+//
+
+import SwiftUI
+
+struct SidebarListStyleModifier: ViewModifier {
+	
+	#if os(iOS)
+	@Environment(\.horizontalSizeClass) private var horizontalSizeClass
+	#endif
+
+	@ViewBuilder func body(content: Content) -> some View {
+		content
+		#if os(macOS)
+		content
+			.listStyle(SidebarListStyle())
+		#else
+		if horizontalSizeClass == .compact {
+			content
+				.listStyle(PlainListStyle())
+		} else {
+			content
+				.listStyle(SidebarListStyle())
+		}
+		#endif
+		
+	}
+
+}