feat(ui-shell): support fixed SideNav

src/UIShell/SideNav/SideNav.svelte

@@ -1,4 +1,10 @@
 <script>
+  /**
+   * Set to `true` to use the fixed variant
+   * @type {boolean} [fixed=false]
+   */
+  export let fixed = false;
+
   /**
    * Specify the ARIA label for the nav
    * @type {string} [ariaLabel]
@@ -12,12 +18,21 @@
   export let isOpen = false;
 </script>
 
+{#if fixed}
+  <div
+    on:click={() => {
+      isOpen = false;
+    }}
+    class:bx--side-nav__overlay={true}
+    class:bx--side-nav__overlay-active={isOpen} />
+{/if}
 <nav
   aria-label={ariaLabel}
   class:bx--side-nav__navigation={true}
   class:bx--side-nav={true}
   class:bx--side-nav--ux={true}
   class:bx--side-nav--expanded={isOpen}
+  class:bx--side-nav--collapsed={!isOpen && fixed}
   {...$$restProps}>
   <slot />
 </nav>