Add $() for inline subcommand substitution that runs a command and inserts its stdout (trailing newlines stripped) into the outer command string. Supports nesting, and mixing with ${expr} and $NAME.
- #25: $VAR expansion in commands - #26: Shell abbreviations and user-extensible builtins - #27: $(cmd) subcommand syntax - #22: Updated with implementation details (in progress) - #7: Standardize status label