Improve Zig AST developer experience

- Convert string-based type discrimination to type-safe enums
  - ContainerKind (struct, enum, union, opaque)
  - LiteralKind (int, float, string, char)
  - LoopKind (for, while)
- Remove duplicate AST nodes (consolidated init lists, removed unused types)
- Add comprehensive helper functions for all AST constructions
- Implement formatters for all AST nodes (expressions, statements, types)
- Add typed literal constructors: IntLit, FloatLit, StringLit, CharLit
- Improve documentation and add deprecation notices

This makes the AST more intuitive and type-safe for developers.

internal/zig/ast.go

@@ -2,9 +2,10 @@
 package zig
 
 // Root is the top-level node of a Zig source file.
+// It represents the entire compilation unit.
 type Root struct {
-	ContainerDocComment DocComment // //! Doc Comment (optional)
-	ContainerMembers    []*ContainerMember
+	ContainerDocComment DocComment         // Module-level doc comment using //!
+	ContainerMembers    []*ContainerMember // Top-level declarations and fields
 }
 
 type ContainerMember struct {
@@ -111,11 +112,21 @@ type ParamDecl struct {
 	Type       TypeExpr // 'anytype' if empty
 }
 
+// ContainerKind represents the kind of container (struct, enum, union, opaque).
+type ContainerKind int
+
+const (
+	ContainerStruct ContainerKind = iota
+	ContainerEnum
+	ContainerUnion
+	ContainerOpaque
+)
+
 // ContainerDecl represents a struct, enum, union, or opaque declaration.
 type ContainerDecl struct {
 	Extern     bool
 	Packed     bool
-	Kind       string   // "struct", "enum", "union", "opaque"
+	Kind       ContainerKind
 	TagType    TypeExpr // Optional (for enum/union)
 	Fields     []*ContainerMember
 	DocComment DocComment
@@ -226,10 +237,18 @@ type ContinueStmt struct {
 
 func (*ContinueStmt) isStmt() {}
 
+// LoopKind represents the kind of loop (for or while).
+type LoopKind int
+
+const (
+	LoopFor LoopKind = iota
+	LoopWhile
+)
+
 // LoopStmt represents a for/while loop statement.
 type LoopStmt struct {
 	Inline  bool       // True if 'inline' is present
-	Kind    string     // "for" or "while"
+	Kind    LoopKind   // For or While
 	Prefix  LoopPrefix // ForPrefix or WhilePrefix
 	Body    Stmt       // Loop body
 	Else    Stmt       // Optional else branch
@@ -331,9 +350,19 @@ type Identifier struct {
 	Name string // The identifier name
 }
 
+// LiteralKind represents the kind of literal.
+type LiteralKind int
+
+const (
+	LiteralInt LiteralKind = iota
+	LiteralFloat
+	LiteralString
+	LiteralChar
+)
+
 // Literal represents a literal value (int, float, string, char).
 type Literal struct {
-	Kind  string // "int", "float", "string", "char"
+	Kind  LiteralKind
 	Value string // The literal value as a string
 }
 
@@ -391,15 +420,14 @@ type NosuspendExpr struct {
 	Expr Expr // The expression to evaluate with nosuspend
 }
 
-// ContinueExpr represents a 'continue' expression.
-type ContinueExpr struct {
-	Label string // Optional label
-}
-
 // Expr is any expression.
+// This is an empty interface to allow maximum flexibility.
+// Consider using type switches when working with expressions.
 type Expr interface{}
 
 // TypeExpr is any type expression.
+// This is an empty interface to allow maximum flexibility.
+// Consider using type switches when working with type expressions.
 type TypeExpr interface{}
 
 // PrefixTypeExpr represents a type with a string prefix. Examples include optionals and pointers.
@@ -500,43 +528,14 @@ type AwaitExpr struct {
 // UnreachableExpr represents the 'unreachable' keyword.
 type UnreachableExpr struct{}
 
-// EmptyInitListExpr represents an empty initializer list '{}'.
-type EmptyInitListExpr struct{}
-
-// PositionalInitListExpr represents a positional initializer list '{expr, ...}'.
-type PositionalInitListExpr struct {
-	Values []Expr // Expressions in order
-}
-
-// FieldInitListExpr represents a field initializer list '{.field = expr, ...}'.
-type FieldInitListExpr struct {
-	Fields []*FieldInit // Field initializers
-}
-
-// SwitchProngPayload represents a switch prong payload (|*x, y|).
-type SwitchProngPayload struct {
-	Pointer bool
-	Names   []string
-}
-
-// SwitchProngCase represents a single case in a switch prong.
-type SwitchProngCase struct {
-	Expr Expr // The case expression
-	To   Expr // Optional, for ranges
-}
-
-// SwitchProngFull represents a full switch prong with cases and payload.
-type SwitchProngFull struct {
-	Inline  bool
-	Cases   []*SwitchProngCase  // One or more cases
-	Payload *SwitchProngPayload // Optional
-	Expr    Expr                // The result expression
-}
-
-// SwitchElseProng represents an 'else' prong in a switch.
-type SwitchElseProng struct {
-	Expr Expr // The result expression
-}
+// Note: The following types were removed as they were duplicates:
+// - EmptyInitListExpr (use InitListExpr with Empty=true)
+// - PositionalInitListExpr (use InitListExpr with Values)
+// - FieldInitListExpr (use InitListExpr with Fields)
+// - SwitchProngPayload (use Payload)
+// - SwitchProngCase (use SwitchCase)
+// - SwitchProngFull (use SwitchProng)
+// - SwitchElseProng (use SwitchProng with SwitchCase.IsElse=true)
 
 // VarPattern represents a variable pattern for destructuring or multiple variable declarations.
 type VarPattern struct {