add control structures: if, for, while, until

- new parser (parser.go) for lists, pipelines, ! and if/elif/else/fi,
  for ... in ... do/done, while/until ... do/done; simple commands are
  kept as text and expanded when they run (in every iteration)
- break [n], continue [n] and the : builtin
- incomplete input (open control structure, trailing &&, |, quote or
  backslash) continues on the next line: prompt "> " in interactive
  shells, scripts and startup files read on
- Ctrl+C aborts the whole command line, including loops
- aliases are expanded at the start of every simple command, also inside
  control structures
- control structures in pipelines, with redirections or in the
  background are reported as not supported
- completion: commands after if, then, do etc.
- main_test.go reformatted with gofmt

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

alias.go

@@ -75,7 +75,7 @@ func expandAliasesIn(line string, active map[string]bool) string {
 	for i := 0; i < len(line); i++ {
 		ch := line[i]
 		if quote == 0 && !escaped && commandPos {
-			if ch == ' ' || ch == '\t' {
+			if ch == ' ' || ch == '\t' || ch == '\n' {
 				out.WriteByte(ch)
 				continue
 			}