Add ESLint rule to require that relative imports use full extensions #753

Job	Run time
Slow tests: defaults with npm	1m 22s
Slow tests: defaults with yarn	1m 15s
Tests: Type Check	16s
Lint JS	21s
Tests (addon-location)	53s
Slow tests: defaults with pnpm	41s
Tests (test-app-location)	1m 13s
Tests (addon-only)	25s
Tests (monorepo with npm)	1m 47s
Tests (monorepo with yarn)	1m 18s
Tests (monorepo with pnpm)	1m 7s
Slow tests: typescript with npm	1m 30s
Tests (rollup-build)	1m 4s
Slow tests: typescript with yarn	1m 16s
Tests (declarations-configuration)	39s
Slow tests: typescript with pnpm	48s
	15m 55s

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