feat(schematics): add createEffect migration

[timdeschryver]

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?

Closes #

What is the new behavior?

This PR adds a new schematic create-effect-migration to migrate from the @Effect() decorator to the createEffect() function.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 991e44b at https://previews.ngrx.io/pr2136-991e44b/

[ngrxbot]

Preview docs changes for 4b20b2c at https://previews.ngrx.io/pr2136-4b20b2c/

[dummdidumm]

I don't see a test for

@Effect()
effectAsMethod() {
   return this.actions$...;
}

Did you check that this gets migrated, too? Or will these intentionally not get migrated?

[timdeschryver]

Thanks @dummdidumm !
I haven't thought of it, I usually see the syntax covered in the PR.

[evgenyfedorenko]

Also maybe as a part of this PR information about this migration should be added to the documentation?

[timdeschryver]

⚠ Do not merge now ⚠

There's an issue with this migration, the createEffect function can be imported for a second time if it's already been imported before the schematic has run.

[ngrxbot]

Preview docs changes for 9e630ee at https://previews.ngrx.io/pr2136-9e630ee/

[timdeschryver]

Removing the WIP tag, I verified it works now against our code base.