Add config to disable the default File loader

[theory]

I'm working on a postgres extension using boon, and I'd like to prevent it from loading the default file loader. Right now I'm replacing it with a no-op loader, but it'd be nice to have a feature to enable to just prevent it from being compiled. Something to complement the cfg(not(target_arch = "wasm32")) macro, having the same behavior but not specific to wasm.