Internal refactor to use Flow (beta release)
All primary src files have been type-annotated with Flow. This should be an invisible change to both end users and developers, but because it was a fairly substantial update, it's being trialled as a beta.
@rhys-vdw @yp @haste @AlexanderOtavka If you guys have a sec, could you please install 2.9.9-beta1 and confirm that everything works properly? :)
Huge thanks to @Hypnosphi for their work in this update!