Map tree collapsed by default

[thetarnav]

Current approach for mapping the tree splits it by roots and only maps roots that could change. But it maps it all, even if it's not visible by the user.

The problem is that the reactivity structure grows very quickly and it easily becomes impossible to navigate without "jumping to nodes" (eg. by searching or locating it on the page).

Having the structure collapsed by default could offer a couple of benefits, but it probably would mean having to refactor the whole logic, both in the debugger and frontend.
Maybe there is an option to test the ux beforehand.

If rewritten, debugger would have to keep track of expanded nodes, even if they get deleted. Every new added node would be collapsed by default.

This would allow us to walk only the expanded part of the tree, instead of splitting walks by roots.

[thetarnav]

I'm adding additional tree-view modes instead: #189