feat(store): make reducers accessible from ReducerManager

[GregOnNet]

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PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The ActionReducerMap of ReduceManager is internal.
It is hard to enhance ActionReducerMap dynamically.

Closes #3049

What is the new behavior?

The ActionReducerMap of ReduceManager can be accessed via getter (READ-ONLY).
This allows to get to know which features are registered in the store, at run-time.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 7636216 at https://previews.ngrx.io/pr3064-76362166/

[brandonroberts]

I'd rather not use snapshot here

Suggested change

	get reducerSnapshot(): ActionReducerMap<any, any> {
	get currentReducers(): ActionReducerMap<any, any> {

[GregOnNet]

Thanks for the suggestion.
I will apply it.

I came up with snapshot, since Angular uses this term, too (see ActivatedRoute) and I wanted to reuse already known terms.

But I am fine with currentReducers, too.