refactor(example): use barrel import for core module

[jasonhodges]

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?

relates to #1808

Closes #

What is the new behavior?

This PR introduces barrel imports for core module and its sub-folders.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 82eeba8 at https://previews.ngrx.io/pr1869-82eeba8/

[ngrxbot]

Preview docs changes for 47b1853 at https://previews.ngrx.io/pr1869-47b1853/

[timdeschryver]

LGTM, thanks @jasonhodges 👍