chore: update custom-field dev page, fix console error (#9625)

Author: web-padawan

dev/custom-field.html

@@ -5,6 +5,7 @@
     <meta http-equiv="X-UA-Compatible" content="IE=edge" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Custom Field</title>
+    <link rel="stylesheet" href="/packages/vaadin-lumo-styles/lumo.css" />
     <script type="module" src="./common.js"></script>
   </head>
 
@@ -17,7 +18,7 @@
     >
       <vaadin-select style="width: 6rem" value="+358" required></vaadin-select>
       <vaadin-text-field
-        allowed-char-pattern="[0-9]*"
+        allowed-char-pattern="[0-9]"
         pattern="[0-9]*"
         maxlength="4"
         style="width: 5rem"