Document %ffi

[fhammerschmidt]

• Add it to syntax lookup
• Adapt https://rescript-lang.org/docs/manual/v12.0.0/embed-raw-javascript to use %ffi instead of %raw

[fhammerschmidt]

The only difference between %raw and %ffi is that with %ffi the arity is checked, so it is safer and should be preferred over raw.

See error in playground.