(core) Get first unicode character (e.g. 📝 ) for page label, not 1st byte

Summary:
📝

https://community.getgrist.com/t/feature-request-custom-icon-favicon-for-each-document/210/2

```
>>> json.dumps("📝")
'"\\ud83d\\udcdd"'
>>> json.dumps("✨")
'"\\u2728"'
```

```
> "📝 abc"[0]
'\uD83D'
> "📝 abc"[1]
'\uDCDD'
> Array.from("📝 abc")
['📝', ' ', 'a', 'b', 'c']
> Array.from("📝 abc")[0]
'📝'
```

Test Plan: no

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3026

app/client/ui2018/pages.ts

@@ -52,7 +52,7 @@ export function buildPageDom(name: Observable<string>, actions: PageActions, ...
       domComputed(isRenaming, (isrenaming) => (
         isrenaming ?
           cssPageItem(
-            cssPageInitial(dom.text((use) => use(name)[0])),
+            cssPageInitial(dom.text((use) => Array.from(use(name))[0])),
             cssEditorInput(
               {
                 initialValue: name.get() || '',
@@ -68,7 +68,7 @@ export function buildPageDom(name: Observable<string>, actions: PageActions, ...
             // firefox.
           ) :
           cssPageItem(
-            cssPageInitial(dom.text((use) => use(name)[0])),
+            cssPageInitial(dom.text((use) => Array.from(use(name))[0])),
             cssPageName(dom.text(name), testId('label')),
             cssPageMenuTrigger(
               cssPageIcon('Dots'),