expand method
Expand variables like $VAR using ARG/ENV definitions.
Implementation
String expand(String value) {
final regex = RegExp(r'\$[A-Za-z_][A-Za-z0-9_]*');
return value.replaceAllMapped(regex, (m) {
final key = m[0]!.substring(1);
if (envs.containsKey(key)) return envs[key]!;
if (args.containsKey(key)) return args[key]!;
return m[0]!; // leave unresolved
});
}