Use .mjs/.cjs extension for runtime files #9385

Job	Run time
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, t...	3m 35s
build-compiler (windows-2025, 5.3.0, true, win32-x64, x86_64-pc-windows-gnu, .exe)	5m 26s
build-compiler (ubuntu-24.04, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, linux...	3m 1s
build-compiler (macos-15, 5.3.0, true, darwin-arm64, aarch64-apple-darwin)	5m 28s
test-devcontainer	5m 15s
build-compiler (macos-15-intel, 5.3.0, true, darwin-x64, x86_64-apple-darwin)	5m 17s
build-compiler (ubuntu-24.04, ocaml-variants.5.0.0+options,ocaml-option-static, linux-x64, x86_64...	2m 42s
pkg-pr-new	1s
api-docs	1s
test-installation-pnpm	1s
test-integration-rewatch	1s
test-installation-npm	1s
publish	1s
	30m 50s

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