Add a scroll event handler that refreshes the view on scroll.

This seems to be a bug in CodeMirror 6.  When scrolling down in a long
document, eventually the text is invisible until you click inside the
editor window.  This event handler refreshes the view on scroll so that
doesn't happen.

package.json

@@ -1,6 +1,6 @@
 {
     "name": "@openwebwork/pg-codemirror-editor",
-    "version": "0.0.1-beta.1",
+    "version": "0.0.1-beta.2",
     "description": "PG CodeMirror Editor",
     "author": "The WeBWorK Project",
     "license": "MIT",

src/pg-codemirror-editor.ts

@@ -141,6 +141,11 @@ export class View {
             ...completionKeymap,
             indentWithTab
         ]),
+        EditorView.domEventHandlers({
+            scroll: () => {
+                this.refresh('scroll-update');
+            }
+        }),
         this.theme.of(lightTheme),
         this.language.of([])
     ];