fix(form-group): add legendId

Ref: 4fc56c30b

src/FormGroup/FormGroup.svelte

@@ -1,31 +1,38 @@
 <script>
+  /** Set to `true` for to remove the bottom margin */
+  export let noMargin = false;
+
   /** Set to `true` to indicate an invalid state */
   export let invalid = false;
 
   /** Set to `true` to render a form requirement */
   export let message = false;
 
-  /** Set to `true` for to remove the bottom margin */
-  export let noMargin = false;
-
   /** Specify the message text */
   export let messageText = "";
 
   /** Specify the legend text */
   export let legendText = "";
+
+  /** Specify an id for the legend element */
+  export let legendId = "";
 </script>
 
 <fieldset
   data-invalid="{invalid || undefined}"
   class:bx--fieldset="{true}"
   class:bx--fieldset--no-margin="{noMargin}"
+  aria-labelledby="{$$restProps['aria-labelledby'] || legendId}"
   {...$$restProps}
   on:click
   on:mouseover
   on:mouseenter
   on:mouseleave
 >
-  <legend class:bx--label="{true}">{legendText}</legend>
+  <legend
+    class:bx--label="{true}"
+    id="{legendId || $$restProps['aria-labelledby']}">{legendText}</legend
+  >
   <slot />
   {#if message}
     <div class:bx--form__requirement="{true}">{messageText}</div>