DEV: adds an experimental `preserveKey` bool option to autocomplete (#12751)

Usage:

```
$(input).autocomplete({
  preserveKey: false
});
```

Defaults to true.

app/assets/javascripts/discourse/app/lib/autocomplete.js

@@ -85,6 +85,10 @@ export default function (options) {
     return this;
   }
 
+  if (options && typeof options.preserveKey === "undefined") {
+    options.preserveKey = true;
+  }
+
   const disabled = options && options.disabled;
   let wrap = null;
   let autocompleteOptions = null;
@@ -199,7 +203,7 @@ export default function (options) {
           let text = me.val();
           text =
             text.substring(0, completeStart) +
-            (options.key || "") +
+            (options.preserveKey ? options.key || "" : "") +
             term +
             " " +
             text.substring(completeEnd + 1, text.length);