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>

subst_test.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"os"
-	"reflect"
 	"testing"
 )
 
@@ -29,17 +28,6 @@ func TestSubstEnd(t *testing.T) {
 	}
 }
 
-func TestSplitList_SkipsSubstitution(t *testing.T) {
-	got, err := splitList(`echo $(a; b && c | d) "$(e || f)" & g`)
-	if err != nil {
-		t.Fatal(err)
-	}
-	want := []listItem{{"", `echo $(a; b && c | d) "$(e || f)" `, true}, {";", " g", false}}
-	if !reflect.DeepEqual(got, want) {
-		t.Errorf("splitList = %+v, want %+v", got, want)
-	}
-}
-
 func TestExpandAliasesAndBraces_SkipSubstitution(t *testing.T) {
 	withAliases(t, map[string]string{"ll": "ls -l"})
 	if got := expandAliases("echo $(ll; ll) && ll"); got != "echo $(ll; ll) && ls -l" {
@@ -106,7 +94,7 @@ func TestExecInput_CommandSubstitution(t *testing.T) {
 	}
 
 	for input, wantErr := range map[string]string{
-		"echo $(echo a":  "invalid input: missing ) in command substitution",
+		"echo $(echo a":   "syntax error: unexpected end of input",
 		"echo $((1 + 2))": "invalid input: arithmetic expansion $((...)) is not supported",
 	} {
 		if err := execInput(input); err == nil || err.Error() != wantErr {