Initialize `_timing` when the user starts typing

inst/shiny-input-app/dev-shiny-input.Rmd

 As a convention,
 the property or method names start with `_`.
 Let's add a `_timing` property that with initialize with `null`.
+(`r github_sha_link("9568fdcd06fa1fe1815dc48bf6efb03f9af68b29")`)
 
 ```js
 $.extend(typingSpeed, {
 ```
 
 </details>
+
+#### Your Turn: Start `_timing`
+
+We're going to use this property to start timing the user's typing.
+On the one hand, 
+when they delete all the text in the text area,
+we want to reset the timers.
+On the other hand,
+when they start typing,
+we want to know when they started.
+
+Update the `.getValue()` method so that whenever
+
+- there are no characters in the textarea
+
+`this._timing` is set to `null` and a `null` value is returned to Shiny.
+
+Similarly, when
+
+- `this._timing` is `null`
+- and there are any characters in the text area
+
+`this._timing` is updated to the current time (`Date.now()`)
+and a `null` value is returned.
+
+Include the timing value in the data returned to Shiny
+so that you can verify it's working.

inst/shiny-input-app/typing.js

     const nchar = el.value.length;
     const nword = el.value.split(' ').length;
 
-    return {nchar, nword};
+    if (nchar === 0) {
+      this._timing = null
+      return null
+    }
+    if (!this._timing && nchar > 0) {
+      this._timing = Date.now()
+      return null
+    }
+
+    return {nchar, nword, timing: this._timing};
   },
   setValue: function(el, value) {
     // This method is used for restoring the bookmarked state of your input