Partially fix slow editor startup (#2382)

The variable 'collapsed' was initialized with the value 'false' because
the initial value of the store 'size' was 'undefined'. This caused an
instance of CodeMirror to be created at editor startup, regardless of
whether the option 'Use HTML editor by default' was enabled, which
significantly slowed down the startup of the editor, especially when
there were many fields in the notetype.

ts/components/Collapsible.svelte

@@ -18,7 +18,7 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
     }
     $: duration = dynamicDuration(contentHeight);
 
-    const size = tweened<number>(undefined);
+    const size = tweened<number>(0);
 
     async function transition(collapse: boolean): Promise<void> {
         if (collapse) {