Improve command coverage

[zac]

Summary

• Add comma brace expansion and simple numeric/alpha ranges.
• Support additional common shell forms and utilities including assignment prefixes, source, set, command, exit, mktemp, shasum, comm, and input process substitution.
• Add a scoped plutil builtin for plist printing, linting, conversion, and narrow top-level replacement.
• Expand regression coverage and docs for the newly covered command and syntax surface.

Testing

• swift test