v0.4.0: Graph-based design.

Graph based design (#5)

* Initial graph-based design with react-flow.

* add scope resize

* pan-on-scroll for touchpad panning

* Use mouse pos for drag; do not drag out of scope.

* Let getScopeAt see up-to-date nodes.

* Do not set parent as dirty when a node moves.

* calculate absolute pos for all ancestors when drop

* sort nodes by level after scope-drop

* fix node.positionAbsolute not found issue

* add monaco editor to CodeNode

* fix navigation-unsave-alert due to react-router-dom v6

* typo: add back update_intervalId

* add execution output

* remove debugging info on signin header