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>

brace.go

@@ -94,8 +94,8 @@ func wordEnd(s string, i int) int {
 				i = j
 			}
 		case ch == '$' && i+1 < len(s) && s[i+1] == '{':
-			if j := strings.IndexByte(s[i:], '}'); j >= 0 {
-				i += j
+			if j := paramEnd(s, i); j >= 0 {
+				i = j
 			}
 		case quote == '"':
 			if ch == '"' {
@@ -134,8 +134,8 @@ func braceScan(s string, fn func(i, depth int) bool) {
 			}
 			continue
 		case ch == '$' && i+1 < len(s) && s[i+1] == '{':
-			if j := strings.IndexByte(s[i:], '}'); j >= 0 {
-				i += j
+			if j := paramEnd(s, i); j >= 0 {
+				i = j
 			}
 			continue
 		case quote == '"':