build: fix up sidenav spacing in M3 dev app

Fixes that there wasn't any side padding in the sidenav when using M3 in the dev app.

Author: crisbeto

src/dev-app/dev-app/dev-app-layout.html

@@ -1,6 +1,6 @@
 <mat-sidenav-container class="demo-container">
   <mat-sidenav #navigation role="navigation">
-    <mat-nav-list>
+    <mat-nav-list class="demo-nav-list">
       @for (navItem of navItems; track navItem) {
         <a
           mat-list-item

src/dev-app/theme-m3.scss

@@ -129,3 +129,8 @@ $density-scales: (-1, -2, -3, -4, minimum, maximum);
 .demo-config-buttons button {
   margin: 4px;
 }
+
+// In M3 we need some spacing around the list in the sidenav.
+mat-nav-list.demo-nav-list {
+  margin: 8px;
+}