feat(router-store): return resolved title via selectTitle

[markostanimirovic]

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
[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?

The selectTitle selector returns only a static title. When custom title resolver is used, the selectTitle result is undefined.

Closes #3622

What is the new behavior?

The selectTitle selector returns resolved route title when a custom title resolver is used.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

BREAKING CHANGES:

Property `title: string | undefined` is added to the `MinimalActivatedRouteSnapshot` interface.

BEFORE:

The `MinimalActivatedRouteSnapshot` interface doesn't contain the `title` property.

AFTER:

The `MinimalActivatedRouteSnapshot` interface contains the required `title` property.

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[timdeschryver]

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