Use `.mjs` for ember-template-lint and Prettier config files #300

Job	Run time
Slow tests: defaults with npm	1m 17s
Slow tests: try-scenarios	2m 4s
Slow tests: defaults with pnpm	41s
Lint	11s
Slow tests: typescript with pnpm	57s
Tests (addon-only)	16s
Slow tests: typescript with npm	1m 28s
Tests (monorepo with npm)	18s
Tests (test-app-location)	17s
Tests (monorepo with pnpm)	17s
Tests (rollup-build)	15s
Tests (addon-location)	15s
Tests (declarations-configuration)	28s
	8m 44s

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