Add ESLint rule to require that relative imports use full extensions #754

Job	Run time
Tests: Type Check	19s
Slow tests: defaults with npm	1m 43s
Lint JS	16s
Tests (addon-location)	47s
Slow tests: defaults with yarn	1m 40s
Tests (test-app-location)	51s
Slow tests: typescript with npm	2m 21s
Slow tests: defaults with pnpm	41s
Tests (addon-only)	25s
Slow tests: typescript with yarn	1m 57s
Tests (monorepo with npm)	1m 31s
Slow tests: typescript with pnpm	1m 18s
Tests (monorepo with yarn)	1m 14s
Tests (monorepo with pnpm)	46s
Tests (rollup-build)	44s
Tests (declarations-configuration)	37s
	17m 10s

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