refactor(Store): don't export internal functions and tokens

[timdeschryver]

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

What is the current behavior?

Closes #1657

What is the new behavior?

Does this PR introduce a breaking change?

[x] Yes
[ ] No

BREAKING CHANGE:

Internal functions and tokens are removed from the public API

Other information

[brandonroberts]

This needs a rebase

[timdeschryver]

@brandonroberts ✔

[ngrxbot]

Preview docs changes for 189008a at https://previews.ngrx.io/pr1679-189008a/