This commit introduces a new subscription in the `routerLinkActive` directive which triggers an update
when any of its associated routerLinks have changes. `RouterLinkActive` not only needs to know when
links are added or removed, but it also needs to know about if a link it already knows about
changes in some way.
Quick note that `from...mergeAll` is used instead of just a simple
`merge` (or `scheduled...mergeAll`) to avoid introducing new rxjs
operators in order to keep bundle size down.
Fixes#18469
PR Close#38349
e0e5c9f195