Fixes addEvent and updateEvent returns (#303)

* addEvent and updateEvent return fix.

* rollup config fix.

packages/core/src/Calendar.svelte

@@ -67,20 +67,23 @@
     }
 
     export function addEvent(event) {
-        $_events.push(createEvents([event])[0]);
+        const ev = createEvents([event])[0]
+        $_events.push(ev);
         $_events = $_events;
-        return this;
+        return ev;
     }
 
     export function updateEvent(event) {
+        let ev = null
         for (let e of $_events) {
             if (e.id == event.id) {
-                assign(e, createEvents([event])[0]);
+                ev = createEvents([event])[0]
+                assign(e, ev);
                 $_events = $_events;
                 break;
             }
         }
-        return this;
+        return ev;
     }
 
     export function removeEventById(id) {

rollup.config.js

@@ -9,7 +9,7 @@ import postcss from 'postcss';
 import autoprefixer from 'autoprefixer';
 import cssnano from 'cssnano';
 import {writeFileSync} from 'fs';
-import pkg from './package.json' assert {type: 'json'};
+import pkg from './package.json' with {type: 'json'};
 import {spawn} from 'child_process';
 
 const production = !process.env.ROLLUP_WATCH;