feat: add autoscroll boolean prop (#1160)

* feat: add autoscroll boolean prop

Fixes #449

* refactor: update autoscroll implementation

Closes #1028
Closes #910

* refactor: only call maybeAdjustScroll in the watcher

* docs: upgrade vuepress

docs/api/props.md

@@ -1,3 +1,16 @@
+## autoscroll <Badge text="v3.10.0+" />
+
+When true, the dropdown will automatically scroll to ensure
+that the option highlighted is fully within the dropdown viewport
+when navigating with keyboard arrows.
+
+```js
+autoscroll: {
+  type: Boolean,
+  default: true
+}
+```
+
 ## appendToBody <Badge text="v3.7.0+" />
 
 Append the dropdown element to the end of the body