configure_file: Add config.vapi support

I think it would be nice if meson could create `config.vapi` files like it can create `config.h` files.

Example
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https://gitlab.gnome.org/GNOME/gnome-mahjongg/blob/master/src/config.vapi

References
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https://mesonbuild.com/Reference-manual.html#configure_file

Start with the configure_file section of the Meson Reference manual and compare its current behavior with the linked GNOME config.vapi example. Trace the existing config.h support, then make configure_file generate config.vapi files with equivalent functionality and document the supported behavior.

Written by the indexing model from the issue text.