Add heif_items API to emscripten #2605

Job	Run time
build (nothing)	57s
build (tarball, 1, 1, 1, 1, 1)	1m 25s
build (graphics, 1)	1m 3s
build (x265, 1)	1m 13s
build (x265 / graphics, 1, 1)	1m 5s
build (libde265 (1) / graphics, 1, 1)	1m 10s
build (libde265 (2) / graphics, 1, 2)	2m 33s
build (libde265 (1) / x265 / graphics, 1, 1, 1)	1m 24s
build (libde265 (2) / x265 / graphics, 1, 1, 2)	2m 44s
build (libde265 (1) / x265 / aom / graphics, 1, 1, 1, 1)	1m 34s
build (libde265 (1) / x265 / dav1d / graphics, 1, 1, 1, 1)	2m 17s
build (libde265 (1) / x265 / aom / rav1e / graphics, 1, 1, 1, 1, 1)	10m 50s
	28m 15s

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