refactor(formula): extract formula parser into separate crate

Extract the formula AST and parser into a dedicated `improvise-formula`
crate and convert the project into a Cargo workspace.

The root crate now re-exports `improvise-formula` as `crate::formula` to
maintain backward compatibility for internal callers. The repository map is
updated to reflect the new crate structure.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

crates/improvise-formula/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "improvise-formula"
+version = "0.1.0-rc2"
+edition = "2024"
+description = "Formula parser and AST for improvise"
+license = "Apache-2.0"
+repository = "https://github.com/fiddlerwoaroof/improvise"
+
+[dependencies]
+anyhow = "1"
+serde = { version = "1", features = ["derive"] }