Add referenceTarget to ShadowRootInit argument to attachShadow()

From review feedback in the Chromium implementation CL, it may be good to add `referenceTarget` to the `ShadowRootInit` dictionary argument to `attachShadow()`. https://chromium-review.googlesource.com/c/chromium/src/+/5615615/comment/1a3651e1_4b117a02/