feat(store): add default generic type to Store and MockStore

[wesleygrimes]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

A common pattern has emerged in NgRx usage where types are no longer being provided to the Store injection point, and as such, most code is just getting an empty object {} for the generic. The type safety is nominal as we are already using Selectors and suggesting that selectors are the preferred way to select information from the store. Selectors themselves have types, so it is nice to be able to use the Store without providing redundant, and often unused typing.

Adding a default to the Store and MockStore class generics, so that Store and MockStore can be injected without the need for providing a type.

[ ] 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?

Currently you must provide a generic type to the Store class:

export class MyAppComponent {
  counter: Observable<number>;

  constructor(private store: Store<AppState>) {
    this.counter = store.pipe(select(fromCounter.selectCounter));
  }
}

Closes #

What is the new behavior?

After this change you can do as follows:

export class MyAppComponent {
  counter: Observable<number>;

  constructor(private store: Store) {
    this.counter = store.pipe(select(fromCounter.selectCounter));
  }
}

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[wesleygrimes]

I would like some feedback on what documentation should be updated? Should we go through all the examples are update them to reflect this new usage of Store?

[ngrxbot]

Preview docs changes for 2e34f62 at https://previews.ngrx.io/pr2325-2e34f62/

[timdeschryver]

👍
For docs, I think the usage of Store<T> is minimal - the same counts for the guides.
The example app can benefit from a refactor imho, but this can be done in a separate PR.

[wesleygrimes]

Do you want me to update the usage or leave as is for the docs?

[timdeschryver]

I would leave it as is where selectors are not used. What about adding a note on where to (not) use generics?

[alex-okrushko]

+1
I'd add that if you are using createSelector / createFeatureSelector from the top level and down - then it's safe to drop the generic.

[wesleygrimes]

@alex-okrushko @timdeschryver thoughts on the latest commit? Updated the docs

[wesleygrimes]

Based on feedback, added a section to the selectors guide on using the store default generic. Any tweaks? Or is this good?
cc @timdeschryver @alex-okrushko