Mark `js-sys` as optional for identity_core #5069

Job	Run time
check-for-run-condition	1s
check-for-modification	3s
build-wasm / build-wasm (ubuntu-latest)	1m 0s
build-and-test (ubuntu-latest)	18m 2s
build-and-test (macos-latest)	7m 6s
build-and-test (windows-latest)	21m 11s
test-wasm	0s
test-wasm-firefox	0s
test-wasm-chrome	0s
	47m 23s

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