refactor: declare inputMode as property in big decimal field

Set inputMode via a Polymer property declaration instead of the
ready() override that set the attribute imperatively on the input
element.

Author: vursen

vaadin-text-field-flow-parent/vaadin-text-field-flow/src/main/resources/META-INF/frontend/vaadin-big-decimal-field.js

@@ -48,15 +48,15 @@ class BigDecimalField extends TextField {
         value: '.',
         sync: true,
         observer: '__decimalSeparatorChanged'
+      },
+
+      inputMode: {
+        type: String,
+        value: 'decimal'
       }
     };
   }
 
-  ready() {
-    super.ready();
-    this.inputElement.setAttribute('inputmode', 'decimal');
-  }
-
   __decimalSeparatorChanged(separator, oldSeparator) {
     this.allowedCharPattern = '[-+\\d' + separator + ']';