LibWeb: Implement HTMLTableRowElement.insertCell

Author: https://github.com/Lubrsi
Commit: 6c9b3fb62e
Pull-request: https://github.com/SerenityOS/serenity/pull/15964
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/HTMLTableRowElement.idl

@@ -1,5 +1,6 @@
 #import <DOM/HTMLCollection.idl>
 #import <HTML/HTMLElement.idl>
+#import <HTML/HTMLTableCellElement.idl>
 
 // https://html.spec.whatwg.org/multipage/tables.html#htmltablerowelement
 [Exposed=Window]
@@ -16,5 +17,5 @@ interface HTMLTableRowElement : HTMLElement {
     readonly attribute long sectionRowIndex;
 
     [SameObject] readonly attribute HTMLCollection cells;
-
+    HTMLTableCellElement insertCell(optional long index = -1);
 };