feat(store): Expose On in public api

[JohnYoungers]

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?

[ ] 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 if you need to import On (to wrap or create your own createReducer), it will be imported from @ngrx/store/src/reducer_creator

Closes #

What is the new behavior?

On will be imported from @ngrx/store

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

[ngrxbot]

Preview docs changes for 7bbed46 at https://previews.ngrx.io/pr2157-7bbed46/