Provide a lightweight runtime crate to remove all the build time dependencies

[tisonkun]

Currently, the idiom to use shadow-rs is depending on it for both build-dependencies and dependencies.

However, the code necessary in build time is not always used in runtime, such as git2, tzdb, extra.

I suppose we can provide a runtime-only crate for users.

I currently try inline the shadow! macro:

pub mod build {
    include!(concat!(env!("OUT_DIR"), "/shadow.rs"));
}

and get the compile error:

    |
252 | pub const VERSION:&str = shadow_rs::formatcp!(r#"
    |                          ^^^^^^^^^ use of unresolved module or unlinked crate `shadow_rs`

So perhaps we can provide a shadow-lib crate that contains only the macro and reexport formatcp!?

cc @baoyachi

[tisonkun]

Work it around now with:

pub mod build {
    mod shadow_rs {
        pub use const_format::formatcp;
    }

    include!(concat!(env!("OUT_DIR"), "/shadow.rs"));
}