refactor: Replace individual type checks for customFlagProperty #1457

Job	Run time
Build Distribution Bundle	36s
Test Jest (16.x)	27s
Test Jest (17.x)	36s
Test Jest (18.x)	29s
Test Jest (19.x)	31s
Test Jest (20.x)	29s
Get Bundle Size from Current Branch	23s
Get Bundle Size from Master	6s
Core Tests / Core SDK Tests	1m 11s
Calculate Bundled Diff / Calculate Bundle Size Difference	8s
Generate Report	1s
Test Stub	39s
Integration Tests: webpack Common JS / webpack CJS Test	1m 3s
Integration Tests: Rollup Module JS / Rollup Module Test	1m 12s
Integration Tests: Rollup Common JS / Rollup CJS Test	1m 5s
Integration Tests: Browserfy Common JS / Browserfy CJS Test	1m 6s
Integration Tests: webpack Module JS / webpack Module Test	1m 8s
Integration Tests: Require JS / Require JS Test	43s
Save PR Number for Dependabot Automerge / Save PR Number for Dependabot Automerge	0s
	11m 53s

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