feat: add diagnostic system and improve CLI with check command

Implement comprehensive diagnostic reporting system:
- Add Diagnostic struct with severity levels and span-based error tracking
- Add diagnostic rendering with source context and caret positioning
- Replace ParseError with diagnostic collection in lexer and parser
- Add LexResult and ParseResult types to carry diagnostics

Enhance driver crate with frontend output:
- Replace CompileResult with FrontendOutput containing diagnostics
- Add has_errors() and render_diagnostics() methods
- Add AstSummary for

crates/nxc-cli/Cargo.toml

@@ -5,6 +5,10 @@ edition.workspace = true
 license.workspace = true
 authors.workspace = true
 
+[[bin]]
+name = "nxc"
+path = "src/main.rs"
+
 [[bin]]
 name = "nexacore"
 path = "src/main.rs"