Add ESLint rule to require that relative imports use full extensions #755

Job	Run time
Tests: Type Check	17s
Tests (addon-location)	48s
Lint JS	22s
Tests (test-app-location)	51s
Slow tests: defaults with npm	1m 35s
Tests (addon-only)	27s
Slow tests: defaults with yarn	1m 16s
Tests (monorepo with npm)	1m 27s
Slow tests: typescript with npm	2m 55s
Slow tests: defaults with pnpm	43s
Tests (monorepo with yarn)	1m 15s
Slow tests: typescript with yarn	1m 53s
Tests (monorepo with pnpm)	47s
Slow tests: typescript with pnpm	1m 46s
Tests (rollup-build)	1m 0s
Tests (declarations-configuration)	39s
	18m 1s

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