fix json schema for angular.json to allow 'targets' property

[zhuravlikjb]

Currently a newly created angular-cli project contains angular.json not passing checks with the json schema bundled with corresponding package:

The issue was reported by WebStorm QA as https://youtrack.jetbrains.com/issue/WEB-34738.

This PR fixes the issue, allowing 'targets' property.

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[zhuravlikjb]

signed the CLA

[googlebot]

CLAs look good, thanks!

[alan-agius4]

Thank for your contribution but there is already a PR for this #12204

[crashsol]

signed the CLA

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