Merge C++ templates together

Looking [at a profile today](https://share.firefox.dev/3Nvagli), and I realized I would kind of love the ability to merge by C++ template base class.

E.g. for the search above: I actually am much much less interested in the difference between `JS::Rooted::registerWithRootLists` and `JS::Rooted::registerWithRootLists` as I am interested in the holistic view of -all- calls to this, across all types.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-965)