Added data validation for the input box (+3 squashed commit)

Squashed commit:

[1a39c41fdfe] Added clientside validation

[b3f97d9a78d] Added PHP Doc block for class

[fdb75fff538] error message added if qty is left empty

Author: konarshankar07

app/code/Magento/Multishipping/view/frontend/templates/checkout/addresses.phtml

@@ -62,8 +62,8 @@
                                            name="ship[<?= $block->escapeHtml($_index) ?>][<?= $block->escapeHtml($_item->getQuoteItemId()) ?>][qty]"
                                            value="<?= $block->escapeHtml($_item->getQty()) ?>"
                                            size="2"
-                                           class="input-text qty"
-                                           data-validate="{number: true}"/>
+                                           class="input-text qty required-entry"
+                                           data-validate="{number: true, required:true}"/>
                                 </div>
                             </div>
                         </td>