Invalid multiline text node indentation

Currently for input:

```

My text
Another line

The output is:

```

My text
Another line

so first line is properly formatted (indentation corrected), but following lines are not.

This is an issue for me, because we are building HTML using different template strings, helpers etc, so indentation is essentially unpredictable. We use `diffable-html` in tests to compare expected HTML with actual HTML, and both expected and actual are built from template strings. Therefore, we need proper normalization.

I created patch that works for me:

```
diff --git a/dist/diffable-html.js b/dist/diffable-html.js
index c732d9e3eca544b70f1d10c8291881e3a5925c60..c4978caa2cbcc1064e33c65668feb86c22802fb7 100644
--- a/dist/diffable-html.js
+++ b/dist/diffable-html.js
@@ -206,9 +206,17 @@ var format = function(html, ref) {
return;
}

- appendLineBreak();
- appendCurrentIndentation();
- append(trimmed);
+ const splitted = text.split('\n');
+ for (const line of splitted) {
+ const lineTrimmed = line.trim();
+ if (lineTrimmed.length === 0) {
+ continue;
+ }
+
+ appendLineBreak();
+ appendCurrentIndentation();
+ append(lineTrimmed);
+ }
},
onclosetag: function(tagname) {
var isVoidTag = isVoidTagName(tagname);

```

Essentially it uses current logic for `onText` but applies it to each line. It seems it fixed my problem.
Note: this patch removes empty lines in text nodes.

報告された動作は dist/diffable-html.js の onText callback で処理されています。まずその handler と周辺のフォーマット処理を読んでください。提示された複数行の入力を再現し、各行が意図どおりに扱われていることを確認してください。空行を保持するべきか削除するべきかも含めて確認します。

索引モデルが issue の本文から書いたものです。