🐛(frontend) fix emojipicker closing

In the tree view, if the emoji picker is opened
near the bottom of the viewport, it would
trigger an overflow that rerendered the treeview
and closed the picker immediately.
The root problem is the treeview that rerender
because of not stable props.
To fix this, we change 2 things:
- we use "fixed" position for the emoji picker
  so it won't affect the document flow
- we adjust the position calculation logic, if
  the picker does not have enough space below,
  we position it above the icon instead.

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to
 
 - ✅(e2e) fix e2e test for other browsers #1799
 - 🐛(frontend) add fallback for unsupported Blocknote languages #1810
+- 🐛(frontend) fix emojipicker closing in tree #1808
 
 ### Changed