feat: export gix_validate as validate.

The crate appears in the public API of `gix_index::State::from_tree`
but it's types weren't readily available.

gix/src/lib.rs

@@ -110,6 +110,7 @@ pub use gix_features::{
     progress::{Count, DynNestedProgress, NestedProgress, Progress},
     threading,
 };
+pub use gix_validate as validate;
 pub use gix_fs as fs;
 pub use gix_glob as glob;
 pub use gix_hash as hash;