feat(router-store): add createRouterSelector to select router data for default config

[timdeschryver]

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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?

Closes #

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

This is an RFC to make it easier to select the router state when the default router config is used.
Because the router store module is opinionated and uses router as the default feature name, I was thinking about adding a bit of default behavior to the current router selectors. This aims to make it easier to get started with using the router store.

When the default config is used, I think we can:

• export a router state selector
• make getSelectors smarter and to use the default router state selector to retrieve the router state

I started this PR to promote and use the DEFAULT_ROUTER_FEATURENAME (instead of the "magic router" string) constant in the example app and the docs. But then I continued to make the selectors more opinionated, because this is how I've been and seen using it in all the projects.

BEFORE:

export const selectRouter = createFeatureSelector<fromRouter.RouterReducerState>(
  'router'
);

export const { selectRouteData } = fromRouter.getSelectors(selectRouter);

AFTER:

// change 1, select the default router state with createRouterSelector

export const selectRouter = createRouterSelector();

// change 2, the selector argument of getSelectors isn't required

export const { selectRouteData } = fromRouter.getSelectors();

[markostanimirovic]

I like the idea, Tim 👍 Just a small suggestion:

[ngrxbot]

Preview docs changes for 620987c at https://previews.ngrx.io/pr3103-620987c1/