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_test.go

@@ -15,6 +15,10 @@ func resetShell(t *testing.T) {
 	t.Cleanup(func() {
 		functions = map[string]*funcDef{}
 		positional, scriptName = nil, "gosh"
+		errexit, xtrace, pipefail = false, false, false
+		for name := range traps {
+			setTrap(name, "-")
+		}
 	})
 }