Use character display width for table padding

Using len(...) for table cell padding produced wrong results for unicode
chracters leading to misaligned tables. Implementation changed to take
the actual terminal display width into consideration.

internal/ui/format_test.go

@@ -84,3 +84,21 @@ func TestParseBytesInvalid(t *testing.T) {
 		test.Equals(t, int64(0), v)
 	}
 }
+
+func TestTerminalDisplayWidth(t *testing.T) {
+	for _, c := range []struct {
+		input string
+		want  int
+	}{
+		{"foo", 3},
+		{"aéb", 3},
+		{"ab", 3},
+		{"a’b", 3},
+		{"aあb", 4},
+	} {
+		if got := TerminalDisplayWidth(c.input); got != c.want {
+			t.Errorf("wrong display width for '%s', want %d, got %d", c.input, c.want, got)
+		}
+	}
+
+}