Variable expansion and subshells, add Vec<Arg> for commands

[samp5]

Add Support for Subshells + Variable Expansion

• Subshell expansion for cmd1 (cmd2) and cmd1 $(cmd2)
• Variable expansion for $name but not `${name}
• Instead of an Option<Token>, Lexer::lex_subshell returns a Result<Token> so that unmatched parentheses can be collected as errors to emit
• We could implement this for single and double quotes as well.
• Because subshell results should get placed directly into the containing command, I changed Command::argv from a Vec<String> to a Vec<Arg> where Arg is an enum with variants for basic "words", subshells, and variables.

Remove redundant CommandGroup

• Because additional commands are being stored in the and_then field of the Command struct, the commands in CommandGroup was redundant.
• Because of this, AndThen now has to have a Box<Command>

Update Tests

• To test all the new functionality, I basically dropped all the previous tests and (per precedent) had a LLM write a bunch of tests that use the new Vec<Arg> format for Command.

[samp5]

Results for lexing and parsing

• All lexing targets that can produce errors return results.
• The Item for the Lexer iterator implementation is now Result<Token, ParseError>
• If there are any errors during parsing inCommand::parse, a ParseErrors is returned which contains a Vec<ParseErrors>.
  • Each ParseError could be extended to include additional information about the error (as inner values)

Testing

• Instead of rewriting all the tests Ok is mapped to Some and Err to None