Allow selecting multiple call nodes to apply to a call tree transform, or combine them together

I use merge function a ton to flatten out boring parts of a call tree. It would be nice if I could select multiple call nodes to merge them together. I could see clicking on a call node, and the shift clicking on another one. This would select all of the call nodes between, and allow for call tree transforms to target multiple nodes.

The app state could then define the selected call node as an array, and the type system could be used to update everything successfully. This might be a big code diff.

Another option would be for the merge function transform to take multiple call nodes, and if you merged 10 call nodes, it would all be placed into the same entry in the transform.

```
'merge-function': {|
+type: 'merge-function',
+funcIndex: Set,
|},
```

This could help fix the issues in #3760.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-491)