Initial release, extracted from browser_ppx in server-reason-react.
let%browser_only, %browser_only expressions and the [@browser_only] attribute.[@platform js] and [@platform native] attributes.switch%platform with Server/Client and Runtime.Server/Runtime.Client patterns.-js driver flag to preprocess for Melange.ppx_platform.runtime, with the Ppx_platform_runtime module that the generated code calls.