Add YaruNavigationPageTheme.railPadding (#466)

* Add YaruNavigationRailTheme.railPadding

Fixes: #465

* Update goldens

lib/src/layouts/yaru_navigation_page_theme.dart

@@ -7,6 +7,7 @@ class NavigationPageThemeData extends ThemeExtension<NavigationPageThemeData>
     with Diagnosticable {
   /// Creates a theme that can be used with [YaruNavigationPage].
   const NavigationPageThemeData({
+    this.railPadding,
     this.pageTransitions,
   });
 
@@ -16,16 +17,21 @@ class NavigationPageThemeData extends ThemeExtension<NavigationPageThemeData>
     );
   }
 
+  /// The padding around the navigation rail.
+  final EdgeInsetsGeometry? railPadding;
+
   /// The page transitions to use.
   final PageTransitionsTheme? pageTransitions;
 
   /// Creates a copy of this object but with the given fields replaced with the
   /// new values.
   @override
   NavigationPageThemeData copyWith({
+    EdgeInsetsGeometry? railPadding,
     PageTransitionsTheme? pageTransitions,
   }) {
     return NavigationPageThemeData(
+      railPadding: railPadding ?? this.railPadding,
       pageTransitions: pageTransitions ?? this.pageTransitions,
     );
   }
@@ -37,26 +43,30 @@ class NavigationPageThemeData extends ThemeExtension<NavigationPageThemeData>
   ) {
     final o = other as NavigationPageThemeData?;
     return NavigationPageThemeData(
+      railPadding: EdgeInsetsGeometry.lerp(railPadding, o?.railPadding, t),
       pageTransitions: t < 0.5 ? pageTransitions : o?.pageTransitions,
     );
   }
 
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
+    properties.add(DiagnosticsProperty('railPadding', railPadding));
     properties.add(DiagnosticsProperty('pageTransitions', pageTransitions));
   }
 
   @override
   bool operator ==(Object other) {
     if (identical(this, other)) return true;
     return other is NavigationPageThemeData &&
+        other.railPadding == railPadding &&
         other.pageTransitions == pageTransitions;
   }
 
   @override
   int get hashCode {
     return Object.hashAll([
+      railPadding,
       pageTransitions,
     ]);
   }

lib/src/layouts/yaru_navigation_rail.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import 'yaru_navigation_page.dart';
+import 'yaru_navigation_page_theme.dart';
 import 'yaru_navigation_rail_item.dart';
 
 class YaruNavigationRail extends StatelessWidget {
@@ -43,8 +44,9 @@ class YaruNavigationRail extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final theme = YaruNavigationPageTheme.of(context);
     return Padding(
-      padding: const EdgeInsets.symmetric(vertical: 5),
+      padding: theme.railPadding ?? EdgeInsets.zero,
       child: IntrinsicHeight(
         child: Column(
           children: <Widget>[