add parameter expansion, arithmetic, set -e/-x/pipefail, trap, eval, exec

- ${#v}, ${v#p} ${v##p} ${v%p} ${v%%p}, ${v/p/r} ${v//p/r} ${v/#p/r}
  ${v/%p/r}, ${v:off:len}, ${v-w} ${v:=w} ${v:+w} ${v:?msg}; nested ${}
- arithmetic expansion $((...)) with the C operators, ** and assignments
- set -e, set -x, set -o pipefail (also as gosh options, passed on to
  subshells); set shows variables, set -o the options
- trap for EXIT and signals; scripts with an EXIT trap run it on
  SIGINT/SIGTERM/SIGHUP too
- eval, exec (also exec > file without a command), $!, wait pid

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

funcs.go

@@ -31,6 +31,10 @@ var (
 	// positional are the positional parameters $1, $2, ...
 	positional []string
 
+	// lastBackgroundPid is $!: the process ID of the last background job
+	// (of its last process).
+	lastBackgroundPid int
+
 	// funcDepth is the number of functions that are currently running.
 	funcDepth int
 	// localFrames holds, for every running function, the previous values
@@ -156,6 +160,11 @@ func lookupParam(name string) (string, bool) {
 		return strconv.Itoa(len(positional)), true
 	case name == "$":
 		return strconv.Itoa(os.Getpid()), true
+	case name == "!":
+		if lastBackgroundPid == 0 {
+			return "", false
+		}
+		return strconv.Itoa(lastBackgroundPid), true
 	case name == "@" || name == "*":
 		return strings.Join(positional, " "), true
 	case name == "0":