Add ESLint rule to require that relative imports use full extensions #752

Job	Run time
Lint JS	20s
Tests: Type Check	14s
Slow tests: defaults with npm	1m 24s
Tests (addon-location)	44s
Slow tests: defaults with yarn	1m 8s
Slow tests: typescript with npm	1m 30s
Tests (test-app-location)	1m 0s
Slow tests: defaults with pnpm	42s
Slow tests: typescript with yarn	1m 12s
Tests (addon-only)	25s
Slow tests: typescript with pnpm	39s
Tests (monorepo with npm)	1m 27s
Tests (monorepo with yarn)	1m 8s
Tests (monorepo with pnpm)	49s
Tests (rollup-build)	2m 10s
Tests (declarations-configuration)	39s
	15m 31s

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