ESLint 9 + flat configs #847

Job	Run time
Tests: Type Check	17s
Lint JS	18s
Slow tests: defaults with npm	21s
Tests (addon-location)	45s
Slow tests: defaults with yarn	1m 25s
Tests (test-app-location)	47s
Slow tests: typescript with npm	25s
Slow tests: defaults with pnpm	1m 0s
Tests (addon-only)	24s
Slow tests: typescript with yarn	1m 8s
Tests (monorepo with npm)	26s
Slow tests: typescript with pnpm	45s
Tests (monorepo with yarn)	1m 4s
Tests (monorepo with pnpm)	58s
Tests (rollup-build)	58s
Tests (declarations-configuration)	36s
	11m 37s

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