Implement parser and executor support for pipelines and redirects

[ElianThorne]

• extended lexer/parser to recognize shell words, command arguments, pipes, and redirects
• added AST nodes for Command, Pipeline, and Redirect with redirect modes
• wired execution in main.rs for pipelines and file redirection
• preserved fallback to system command execution on parse fallback
• verified with cargo test

[AkshajSinghal]

Continue working on it, looks good!