fix(editor): do not prevent shift+enter to add a line break in text

Resolves https://github.com/go-vikunja/vikunja/issues/250

frontend/src/components/input/editor/TipTap.vue

@@ -338,10 +338,12 @@ const editor = useEditor({
 		HardBreak.extend({
 			addKeyboardShortcuts() {
 				return {
+					'Shift-Enter': () => this.editor.commands.setHardBreak(),
 					'Mod-Enter': () => {
 						if (contentHasChanged.value) {
 							bubbleSave()
 						}
+						return true
 					},
 			    }
 			},