simplify copy-n-paste

[lihebi]

New behavior:

1. select a pod,
2. ctrl-c to copy, and
3. ctrl-v to paste at the current mouse position (the same behavior in reactflow's example).

TODOs:

• [x] support copying & pasting multiple pods
• [x] support recursively copying scope pods
• [x] copy connected edges as well
• [x] copy the content as well, both code and rich pods
• [ ] highlight newly copied pods to indicate that it's not moved into a scope yet
• [ ] support copying results

[lihebi]

Merging, leaving the rest TODOs in follow-up PRs.