Split active trail handling from MenuTree(Interface) into MenuActiveTrail(Interface)

Problem/Motivation

I'm working on #1805054: Cache localized, access filtered, URL resolved, and rendered menu trees, and this is a soft blocker (hence this is marked major, because #1805054 is marked critical).

In #1805054, I have to make quite a few changes to MenuTree. But all of the active trail handling in there is unaffected… precisely because active trail handling is completely independent of dealing with menu trees!
The active trail is only used in some cases to help build the three, but the concept of "active trail" is:

• independent of building a menu tree itself
• a concept that some sites may want to override because it's slowing their website down unnecessarily

MenuTree is confusing enough as is, even without the active trail handling. Let's make MenuTree only about handling… menu trees.

Remaining tasks

None.

User interface changes

None.

API changes

A new menu_link.active_trail service, with the typical corresponding MenuActiveTrailInterface and MenuActiveTrail class.