Add missing .gitignores to better support --addon-only scenarios (no test app) #708

Job	Run time
Tests: Type Check	14s
Lint JS	20s
[email protected] : defaults with npm	2m 1s
Tests (addon-location)	56s
[email protected] : defaults with yarn	1m 30s
[email protected] : typescript with npm	1m 55s
Tests (test-app-location)	1m 8s
[email protected] : defaults with pnpm	1m 9s
[email protected] : typescript with yarn	1m 37s
Tests (addon-only)	38s
[email protected] : defaults with npm	1m 42s
[email protected] : typescript with pnpm	1m 14s
Tests (monorepo with npm)	1m 29s
[email protected] : defaults with yarn	1m 26s
Tests (monorepo with yarn)	1m 20s
[email protected] : defaults with pnpm	1m 10s
Tests (monorepo with pnpm)	52s
Tests (rollup-build)	1m 3s
Tests (declarations-configuration)	41s
	22m 25s

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