refactor(NumberInputSkeleton): add typescript annotations

Convert existing propType definitions to Typescript annotations on the
NumberInputSkeleton component. This is part of a broader effort to add
Typescript annotations to components, tracked in carbon-design-system#12513. Closes carbon-design-system#12549.

Type annotation changes only; no breaking feature changes.

packages/react/src/components/NumberInput/NumberInput.Skeleton.js → packages/react/src/components/NumberInput/NumberInput.Skeleton.tsx

@@ -6,11 +6,27 @@
  */
 
 import PropTypes from 'prop-types';
-import React from 'react';
+import React, { HTMLAttributes } from 'react';
 import cx from 'classnames';
 import { usePrefix } from '../../internal/usePrefix';
 
-function NumberInputSkeleton({ hideLabel, className, ...rest }) {
+export interface NumberInputSkeletonProps
+  extends HTMLAttributes<HTMLDivElement> {
+  /**
+   * Specify an optional className to add to the form item wrapper.
+   */
+  className?: string;
+
+  /**
+   * Specify whether the label should be hidden, or not
+   */
+  hideLabel?: boolean;
+}
+function NumberInputSkeleton({
+  hideLabel,
+  className,
+  ...rest
+}: NumberInputSkeletonProps) {
   const prefix = usePrefix();
   return (
     <div className={cx(`${prefix}--form-item`, className)} {...rest}>