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.

Il comportamento segnalato viene gestito in dist/diffable-html.js, nel callback onText; inizia leggendo quell’handler e la logica di formattazione che lo circonda. Riproduci l’input multilinea fornito e verifica il trattamento previsto di ogni riga, incluso se le righe vuote debbano essere preservate o rimosse.

Scritto dal modello di indicizzazione a partire dal testo della issue.