Use include_str! to trigger recompile when sample file changes

[evestera]

When reading sample from file using the procedural macro (e.g. json_typegen!("Point", "point.json")), if the sample file changes the compiler will not be aware that anything relevant has changed and so will not recompile.

There is a hack we can do to let the compiler know we are using a file. Output code like this in addition to the actual code:

const _: &'static str = include_str!("point.json");

Tracking issue for better way to do this (from which the snippet above was taken): rust-lang/rust#73921