add command substitution $(...)

- $(command) is replaced by the output of command, which runs in a
  subshell (gosh -c), so cd/set in it do not change the shell
- trailing newlines are removed; unquoted the output is split into words
  at blanks and newlines, "$(...)" stays one word; wildcards in the
  output are not expanded
- nested substitutions, quotes and operators inside are handled; the
  list, alias and brace scanners skip $(...) as a whole
- a command that consists only of a substitution has its exit status,
  Ctrl+C in a substitution aborts the command
- $((...)) is reported as not supported

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

alias.go

@@ -96,6 +96,16 @@ func expandAliasesIn(line string, active map[string]bool) string {
 			}
 		}
 
+		if quote != '\'' && !escaped && isSubstStart(line, i) {
+			// $(...) is copied as it is; the subshell does not expand
+			// aliases
+			if end := substEnd(line, i); end >= 0 {
+				out.WriteString(line[i : end+1])
+				i = end
+				continue
+			}
+		}
+
 		out.WriteByte(ch)
 		switch {
 		case escaped: