Add comprehensive Zig syntax test coverage

- Add support for error sets and error union types
- Implement for loops with index and payload syntax
- Add defer, break, continue, and switch statements
- Support unary expressions and array indexing
- Add unreachable expression and test declarations
- Extend AST with new type expressions (array, error union)
- Update formatter to handle all new syntax elements
- Fix formatting for switch prongs, payloads, and blocks

internal/zig/ast.go

@@ -408,6 +408,23 @@ type PrefixTypeExpr struct {
 	Base TypeExpr
 }
 
+// ArrayTypeExpr represents an array type ([N]T).
+type ArrayTypeExpr struct {
+	Size Expr
+	Elem TypeExpr
+}
+
+// SliceTypeExpr represents a slice type ([]T).
+type SliceTypeExpr struct {
+	Elem TypeExpr
+}
+
+// ErrorUnionTypeExpr represents an error union type (E!T).
+type ErrorUnionTypeExpr struct {
+	ErrSet TypeExpr
+	Type   TypeExpr
+}
+
 // DocComment represents a doc comment (/// or //! lines).
 // Newlines in the string automatically add more comments in the output.
 type DocComment string