feat: add cffi feature

[rockstar]

When building the wasm from flux-lsp, the C extern functions are still
provided as part of the public interface for the wasm, despite the
library not ever using them. By putting this behind a (default) feature
flag, and then depending and this crate without the default features, we
trim about 200k from the final wasm binary size.

[Marwes]

I would suggest moving all the functions/impls/etc that are now under the cffi feature into their own module and then re-export them from the root. That way there is less risk that we add a new extern "C" function and forget to put it under the feature.

#[cfg(feature = "cffi")]
mod cffi;
#[cfg(feature = "cffi")]
pub use cffi::{func1, func2};