Mark `js-sys` as optional for identity_core #5077

Job	Run time
check-for-run-condition	1s
check-for-modification	4s
build-wasm / build-wasm (ubuntu-latest)	4m 58s
build-and-test (ubuntu-latest)	17m 54s
build-and-test (macos-latest)	7m 11s
build-and-test (windows-latest)	18m 59s
test-wasm-chrome (ubuntu-latest)	4m 28s
test-wasm-firefox (ubuntu-latest)	4m 28s
test-wasm (ubuntu-latest)	2m 30s
	1h 0m 33s

Provide feedback