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>

brace.go

@@ -15,7 +15,8 @@ import (
 const maxBraceWords = 100000
 
 // expandBraceWords applies brace expansion to every word of a command (the
-// text of a pipeline). Quoted or escaped braces and ${...} are left alone.
+// text of a pipeline). Quoted or escaped braces, ${...} and $(...) are left
+// alone.
 // A redirection target must expand to a single word.
 func expandBraceWords(input string) (string, error) {
 	var (
@@ -69,6 +70,10 @@ func wordEnd(s string, i int) int {
 			}
 		case ch == '\\':
 			i++
+		case isSubstStart(s, i):
+			if j := substEnd(s, i); j >= 0 {
+				i = j
+			}
 		case ch == '$' && i+1 < len(s) && s[i+1] == '{':
 			if j := strings.IndexByte(s[i:], '}'); j >= 0 {
 				i += j
@@ -103,6 +108,12 @@ func braceScan(s string, fn func(i, depth int) bool) {
 		case ch == '\\':
 			i++
 			continue
+		case isSubstStart(s, i):
+			// The subshell does its own brace expansion
+			if j := substEnd(s, i); j >= 0 {
+				i = j
+			}
+			continue
 		case ch == '$' && i+1 < len(s) && s[i+1] == '{':
 			if j := strings.IndexByte(s[i:], '}'); j >= 0 {
 				i += j