-XJavaScriptFFI does nothing
The extension JavaScriptFFI has not been doing anything since it was added in b372e8ea.
It should either control foreign import javascript
or be deprecated. (For example, the proposed winapi
calling convention https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0277-winapi-callconv.rst#proposed-change does not have a flag.)