feat(effects-ng): export injection tokens

[Char2sGu]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/effects/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have 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?

By provideEffectsManager, we provide an instance of EffectsManager to the token EFFECTS_MANAGER, but the token is not exposed.

When creating utility wrappers upon this library, the EffectsManager instance can be used to register effects, but as the token is not exposed, it can be pretty troublesome to inject the instance.

As an workaround, the user can use the registerEffects function exported from theh @ngneat/effects package, but it makes the code feel un-Angular and it also require the user to check the implementation details.

Issue Number: N/A

What is the new behavior?

The injection tokens are now exported in index so that the user can have access to the provided EffectsManager instance.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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