improve kaptan-spec as a build library and move it to libs dir

Signed-off-by: Onur Özkan <work@onurozkan.dev>

libs/kaptan-spec/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "kaptan-spec"
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+rust-version.workspace = true
+version.workspace = true
+
+[features]
+std = ["dep:syn"]
+
+[dependencies]
+syn = { version = "2", features = ["full"], optional = true }
+
+[dev-dependencies]
+trybuild = "1.0"