fix(store): included underscore in stateNameRegex

[JasonAtallah]

PR Checklist

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• 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?

[x] Bugfix
[ ] 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?

Issue Number: N/A

underscores in state name return an error saying the state name isn't valid because it is not a 'valid object property name'

What is the new behavior?

underscores are allowed in state name

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[splincode]

not sure what this is good

[JasonAtallah]

@splincode, the error isn't logical as an underscore is a valid object property name.

Use case:
When trying to construct a payload through substates for an API call that has underscores in property names (common practice for APIs built in Python), this error adds unnecessary logic for me to find and replace field names before making the request.

[markwhitfeld]

LGTM