Support conditionals

[Maxcode123]

Support conditional statements (if-else).

1. Add production rules in grammar (src/syntax/classical.y)
2. Define AST nodes for conditional statements in src/syntax/ast/nodes.h
3. Implement build methods in src/syntax/ast/builder.h
4. Write unit tests for build methods (tests/unit/ast/test_builder.cpp)
5. Call build methods on production rule actions (src/syntax/classical.y)
6. Write integration test for parser (tests/integration/syntax/test_parser.cpp)