meson: Generate VAPI file

This can be used to call Babl from Vala. This is also very useful when
we want to do the same thing (calling from vala) for GIMP 3.x
2 jobs for wip/nielsdg/generate-vapi in 4 minutes and 1 second (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #409857
latest-meson-lcms

00:04:01

passed #409858
latest-meson-nolcms

00:01:40