feat(store): improved typings for @Select decorator

[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?

[x] Bugfix
[x] 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?

Issue: #875

What is the new behavior?

Correct type checking for selector functions

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[markwhitfeld]

LGTM. Nice work.
Is there anything we need to be aware of regarding the version of TS to support this?
Also, could you add mention of this improvement in the announcement article within this PR.