API: Add $wgAPIFormatModules

The API currently allows for adding action modules with
$wgAPIModules and query modules with various other $wgAPI*Modules
variables. But there's no way to add a formatting module from an
extension. For parallelism, let's add $wgAPIFormatModules to do just
that.

Bug: 57637
Change-Id: Ia0bd253104a4b5bc56d4fe0dc6736e2129b126f3