Add deprecated feature and migrate string_proc to just_fmt

- Add `deprecated` feature flag to Cargo.toml
- Gate `string_proc` module behind both `deprecated` and `string_proc`
  features
- Replace internal `string_proc` dependency with `just_fmt` crate in
  constants macros
- Mark all `string_proc` modules as deprecated with migration
  instructions
- Remove `string_proc` dependency from sheet system

1 files changed, 1 insertions, 1 deletions

diff --git a/src/lib.rs b/src/lib.rs
index a01e5e4..306441b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -63,7 +63,7 @@ pub mod utils {
     }
 
     // Feature `string_proc`
-    #[cfg(feature = "string_proc")]
+    #[cfg(all(feature = "deprecated", feature = "string_proc"))]
     pub mod string_proc {
         extern crate string_proc;
         pub use string_proc::*;