[rust-compiler] Refactor function discovery to use AstWalker visitor

Replaced ~330 lines of hand-written recursive AST traversal with the
existing AstWalker + Visitor infrastructure from react_compiler_ast.

Extended the Visitor trait with:
- 'ast lifetime parameter for storing AST references
- traverse_function_bodies() to skip function body recursion
- enter/leave_variable_declarator for name inference
- enter/leave_call_expression for forwardRef/memo detection
- enter/leave_loop_expression for Babel-compatible scope checks

Created FunctionDiscoveryVisitor that replaces find_functions_to_compile,
visit_statement_for_functions, and find_nested_functions_in_expr with a
single visitor implementation driven by AstWalker::walk_program.