Add snapshot support to `Scripts`

Prior work: https://github.com/GoogleChrome/lighthouse/pull/12770

The new `Scripts` gatherer makes this much easier, but there are some obstacles related to the debugger domain being enabled on multiple gatherers.

This would open the door to having the treemap in snapshot mode as well as `no-unload-listeners`

Review prior work in PR #12770 and inspect the new Scripts gatherer first. Investigate the debugger-domain obstacles caused by enabling it in multiple gatherers. Done means Scripts supports snapshot mode and enables the treemap and no-unload-listeners work described in the issue.

Written by the indexing model from the issue text.