refactor: restructure CLI into library crate and add semantic analysis phase

Restructure nxc-cli crate:
- Extract main logic into lib.rs with main_entry() function
- Move nexacore binary to separate bin/nexacore.rs file
- Keep nxc binary in main.rs as thin wrapper

Add semantic analysis to compilation pipeline:
- Implement semantic analyzer with type checking and name resolution
- Add Type enum with Int, Float, Bool, String, Void, Struct, Function, Error variants
- Add typed AST nodes (TypedModule, TypedFunction

crates/nxc-cli/Cargo.toml

@@ -5,14 +5,16 @@ edition.workspace = true
 license.workspace = true
 authors.workspace = true
 
+[lib]
+path = "src/lib.rs"
+
 [[bin]]
 name = "nxc"
 path = "src/main.rs"
 
 [[bin]]
 name = "nexacore"
-path = "src/main.rs"
+path = "src/bin/nexacore.rs"
 
 [dependencies]
 nxc-driver = { path = "../nxc-driver" }
-