feat(store): expose NgxsModuleOptions as a named type

[splincode]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngxs/store/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
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Now it is not clear how to specify the data type.

What is the new behavior?

The ability to specify in your code what type of data the settings have

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[markwhitfeld]

I have generally been opposed to making objects that are only passed as parameters on the public API into their own thing but I think that this case makes sense.

[markwhitfeld]

This should be a feature because it is adding public api.
Even that it is just giving a name to a type.