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/mod.rs

@@ -1,5 +1,9 @@
 pub mod types;
 pub mod nodes;
+pub mod lexer;
+pub mod parser;
 
 pub use types::*;
 pub use nodes::*;
+pub use lexer::*;
+pub use parser::*;