Add missing .gitignores to better support --addon-only scenarios (no test app) #709

Job	Run time
Lint JS	25s
ember-cli@5.4.1 : defaults with npm	1m 54s
Tests (addon-location)	53s
ember-cli@5.4.1 : defaults with yarn	1m 25s
Tests (test-app-location)	52s
Tests: Type Check	13s
ember-cli@5.4.1 : typescript with npm	2m 19s
ember-cli@5.4.1 : defaults with pnpm	1m 19s
Tests (addon-only)	30s
ember-cli@5.4.1 : typescript with yarn	1m 50s
ember-cli@5.3.0 : defaults with npm	1m 47s
Tests (monorepo with npm)	1m 35s
ember-cli@5.4.1 : typescript with pnpm	2m 17s
ember-cli@5.3.0 : defaults with yarn	1m 26s
Tests (monorepo with yarn)	1m 31s
ember-cli@5.3.0 : defaults with pnpm	1m 1s
Tests (monorepo with pnpm)	53s
Tests (rollup-build)	1m 4s
Tests (declarations-configuration)	43s
	23m 57s

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