feat(textutil): add width-aware text wrapping and clipping

Wrap, Clip, and Pad plain text to a fixed rune width for the terminal panes.

internal/textutil/text_test.go

@@ -0,0 +1,32 @@
+package textutil
+
+import "testing"
+
+func TestClip(t *testing.T) {
+	if got := Clip("abcdef", 4); got != "abc…" {
+		t.Fatalf("clip: %q", got)
+	}
+	if got := Clip("ab", 4); got != "ab" {
+		t.Fatalf("short: %q", got)
+	}
+	if got := Clip("a\nb", 5); got != "a b" {
+		t.Fatalf("newline: %q", got)
+	}
+}
+
+func TestWrap(t *testing.T) {
+	got := Wrap("one two three", 7)
+	want := []string{"one two", "three"}
+	if len(got) != len(want) {
+		t.Fatalf("wrap %#v", got)
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("line %d: %q", i, got[i])
+		}
+	}
+	blank := Wrap("a\n\nb", 10)
+	if len(blank) != 3 || blank[1] != "" {
+		t.Fatalf("blank lines: %#v", blank)
+	}
+}