feat: Implement lexer and parser for AST

Introduces the lexer and parser modules, enabling the conversion of
source code into an Abstract Syntax Tree (AST).

This includes:
- Defining token kinds and structures.
- Implementing lexer logic to tokenize input.
- Defining AST node kinds and structures.
- Implementing parser logic to construct the AST from tokens.
- Adding support for basic expressions, lists, keywords, and
  identifiers.
- Adding the `lazy_static` dependency for keyword interning.
- Refactoring `Value::Nil` to `Value::Void`.

src/ast/nodes.rs

@@ -23,6 +23,8 @@ pub struct Context {
 /// The core AST variant enum for performance and structure
 #[derive(Debug)]
 pub enum UntypedKind {
+    /// The "..." placeholder for incomplete code
+    Nop,
     Constant(Value),
     Identifier(Arc<str>),
     If {
@@ -41,6 +43,7 @@ pub enum UntypedKind {
 impl Node<UntypedKind> {
     pub fn eval(&self, ctx: &mut Context) -> Value {
         match &self.kind {
+            UntypedKind::Nop => Value::Void,
             UntypedKind::Constant(v) => v.clone(),
             UntypedKind::Identifier(_) => todo!("Lookup in Context"),
             UntypedKind::If { cond, then_br, else_br } => {
@@ -49,7 +52,7 @@ impl Node<UntypedKind> {
                 } else if let Some(eb) = else_br {
                     eb.eval(ctx)
                 } else {
-                    Value::Nil
+                    Value::Void
                 }
             }
             UntypedKind::Call { callee, args } => {