Add variables

[hand-burger]

Summary of Changes

This PR introduces support for local variable declarations, read references, and assignment statements.

1. AST Improvements: Extended the AST with Statement::Declare, Statement::Assign, and Expr::Variable nodes to model variables.
2. Parser Upgrades:
   • The parser can now loop and parse multiple statements in a function body instead of requiring exactly one return statement.
   • Parses int x = <expr>; (declarations) and x = <expr>; (assignments).
   • Resolves identifier lookups (x) as expressions.
3. LLVM Codegen:
   • Dynamically allocates stack variables using alloca and stores values using store inside generate_ir.
   • Resolves variable reads by generating load instructions referencing their allocated stack space.
4. Testing: Added unit and end-to-end integration tests verifying local variable compilation and multi-statement function executions.