Use event observers in dropCursor and crossHairCursor

FIX: Make sure the drop cursor is properly cleaned up even when another extension
handles the drop event.

See https://discuss.codemirror.net/t/how-to-remove-the-dropcursor-programmatically/7086

src/dropcursor.ts

@@ -74,7 +74,7 @@ const drawDropCursor = ViewPlugin.fromClass(class {
       this.view.dispatch({effects: setDropCursorPos.of(pos)})
   }
 }, {
-  eventHandlers: {
+  eventObservers: {
     dragover(event) {
       this.setDropPos(this.view.posAtCoords({x: event.clientX, y: event.clientY}))
     },

src/rectangular-selection.ts

@@ -114,7 +114,7 @@ export function crosshairCursor(options: {
       }
     }
   }, {
-    eventHandlers: {
+    eventObservers: {
       keydown(e) {
         this.set(e.keyCode == code || getter(e))
       },

src/tooltip.ts

@@ -340,7 +340,7 @@ const tooltipPlugin = ViewPlugin.fromClass(class {
     }
   }
 }, {
-  eventHandlers: {
+  eventObservers: {
     scroll() { this.maybeMeasure() }
   }
 })