Lexical: Table cell bg and format setting fixes

- Updated table cell background color setting to be stable by
  specifically using the background property over the general styles.
- Updated format shorcuts to be correct header levels as per old editor
  and format menu.
- Updated format changes to properly update UI afterwards.

resources/js/wysiwyg/utils/tables.ts

@@ -282,6 +282,7 @@ export function $clearTableFormatting(table: TableNode): void {
         const cells = row.getChildren().filter(c => $isTableCellNode(c));
         for (const cell of cells) {
             cell.setStyles(new Map);
+            cell.setBackgroundColor(null);
             cell.clearWidth();
         }
     }