fix(@schematics/angular): add `screenshots/` to `.gitignore` #1949

Job	Run time
list	22s
workflow (build-cli)	1m 44s
workflow (devkit-core-tests-rerun)	1m 28s
workflow (devkit-core-tests)	1m 16s
workflow (modules-builder-tests)	1m 44s
workflow (build-unit-tests)	2m 20s
	8m 54s

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