Revert "Remove the imple OptionalSettable for Option<JS<T>>."

This reverts commit 38a4cb1e9f5ea0922f2554aef414dcd715e59dc0.
fixup! Use Cell/RefCell for interior mutability of FormData, BlobData.
fixup! Use Cell/RefCell for interior mutability of DOMImplementation and DOMParser.
fixup! Use Cell/RefCell for interior mutability of DOMImplementation and DOMParser.
fixup! Use Cell/RefCell for interior mutability of FormData, BlobData.
fixup! Use Cell/RefCell for interior mutability of HTMLCollection.
fixup! Use Cell/RefCell for interior mutability of Attr, AttrList, Blob, BrowserContext, ClientRect, and ClientRectList.
fixup! Use Cell/RefCell for interior mutability of FormData, BlobData.
fixup! Use Cell/RefCell for interior mutability of Document.