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>

main_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"os/exec"
@@ -892,6 +893,11 @@ func TestMain(m *testing.M) {
 	// Never read the system's startup files; tests that read the user's
 	// files set $HOME themselves
 	sysConfDir = filepath.Join(dir, "etc")
+	// Command substitution runs gosh, not the test binary
+	subshellPath = filepath.Join(dir, "gosh")
+	if out, err := exec.Command("go", "build", "-o", subshellPath, ".").CombinedOutput(); err != nil {
+		panic(fmt.Sprintf("building gosh: %v\n%s", err, out))
+	}
 	code := m.Run()
 	os.RemoveAll(dir)
 	os.Exit(code)