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, 0 deletions

diff --git a/Cargo.toml b/Cargo.toml
index ab2e5eb..d2d44d5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -22,6 +22,7 @@ sha1_hash = []
 tcp_connection = []
 string_proc = []
 lib = []
+deprecated = []
 
 [workspace]
 members = [