fix(material/menu): prevent divider styles from bleeding out

Fixes that the styles for the menu divider were at the top level which means that they'll also affect dividers outside of the menu.

Fixes #29106.

src/material/menu/menu.scss

@@ -63,17 +63,17 @@ mat-menu {
   @include cdk.high-contrast(active, off) {
     outline: solid 1px;
   }
-}
 
-.mat-divider {
-  // Use margin instead of padding since divider uses border-top for line
-  @include token-utils.use-tokens(
-    tokens-mat-menu.$prefix,
-    tokens-mat-menu.get-token-slots()
-  ) {
-    color: var(#{token-utils.get-token-variable(divider-color)});
-    margin-bottom: var(#{token-utils.get-token-variable(divider-bottom-spacing)});
-    margin-top: var(#{token-utils.get-token-variable(divider-top-spacing)});
+  .mat-divider {
+    // Use margin instead of padding since divider uses border-top to render out the line.
+    @include token-utils.use-tokens(
+      tokens-mat-menu.$prefix,
+      tokens-mat-menu.get-token-slots()
+    ) {
+      color: var(#{token-utils.get-token-variable(divider-color)});
+      margin-bottom: var(#{token-utils.get-token-variable(divider-bottom-spacing)});
+      margin-top: var(#{token-utils.get-token-variable(divider-top-spacing)});
+    }
   }
 }