JSX PPX: add Jsx.element return constraint #8686

Job	Run time
build-compiler (ubuntu-24.04, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, linux...	3m 14s
build-compiler (ubuntu-24.04, ocaml-variants.4.14.2+options,ocaml-option-static, linux-x64, x86_6...	4m 22s
build-compiler (windows-2022, 5.3.0, true, win32-x64, x86_64-pc-windows-gnu)	4m 55s
build-compiler (macos-15, 5.3.0, true, darwin-arm64, aarch64-apple-darwin)	5m 24s
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, t...	4m 16s
build-compiler (macos-15-intel, 5.3.0, true, darwin-x64, x86_64-apple-darwin)	8m 23s
pkg-pr-new	52s
api-docs	7s
test-installation-npm (macos-15)	24s
test-installation-npm (ubuntu-24.04-arm)	17s
test-installation-npm (ubuntu-24.04)	19s
test-installation-pnpm (ubuntu-24.04)	16s
test-installation-npm (macos-15-intel)	31s
test-installation-pnpm (macos-15)	14s
test-integration-rewatch (macos-15-intel)	2m 23s
test-installation-npm (windows-2022)	43s
test-integration-rewatch (ubuntu-24.04)	1m 21s
test-installation-pnpm (windows-2022)	32s
test-installation-pnpm (macos-15-intel)	31s
test-integration-rewatch (windows-2022)	2m 16s
test-integration-rewatch (ubuntu-24.04-arm)	1m 5s
test-installation-pnpm (ubuntu-24.04-arm)	19s
test-integration-rewatch (macos-15)	1m 32s
publish	-1s
	44m 15s

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