backticks: support brace expansion in command literals
Implement bash-style brace expansion in backtick Cmd syntax.
`{a,b,c}` expands to three separate arguments, and brace groups
participate in the existing Cartesian product logic so that
`pre{a,b}.{x,y}suf` produces all four combinations. Alternatives
support `$` interpolation, quoting, and escape sequences. Braces
without a comma (`{a}`, `{}`) are treated as literal characters,
matching bash semantics.
Unquoted whitespace inside braces is an error, following zsh.
Quoted spaces like `{"a b",c}` work as expected.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
