🐛 Fix inserting newlines into formatted text

[alecgibson]

Consider a document with a single piece of bold text. As a Delta, this
would be represented as:

const doc = new Delta().insert('a', {bold: true})

Now consider the following Delta being applied:

const delta = new Delta().insert('\n1')

The above Delta will:

• prepend the document with a newline
• follow that newline with an unformatted string

Indeed, using doc.compose(delta) yields:

const result = new Delta()
  .insert('\n1')
  .insert('a', {bold: true})

However, the same result is not reached when using the Quill
editor.applyDelta(), which instead results in bold formatting being
incorrectly applied to our unformatted string:

const badResult = new Delta()
  .insert('\n')
  .insert('1a', {bold: true})

This happens because Quill does an insert of the whole insertion, but
doesn't handle the line splitting.

This change fixes this issue by splitting ops on newlines, and handling
them separately, which applies the correct formatting.

[luin]

Hey @alecgibson 👋

Good catch! I just started working on this. Sorry for the delay! Haven't looked into the reason but it appears that the code fails the following test case:

it('#3582', function () {
  const editor = this.initialize(
    Editor,
    '<iframe class="ql-video" frameborder="0" allowfullscreen="true" src="#" width="100" height="300"></iframe><p><br></p>',
  );
  editor.applyDelta(
    new Delta()
      .insert('\n')
      .insert({ video: '#' }, { width: '100', height: '300' }),
  );
  expect(this.container).toEqualHTML(
    '<p><br></p><iframe class="ql-video" frameborder="0" allowfullscreen="true" src="#" width="100" height="300"></iframe><iframe class="ql-video" frameborder="0" allowfullscreen="true" src="#" width="100" height="300"></iframe><p><br></p>',
  );
});```

[alecgibson]

@luin I've pushed a fix. Is there a reason that test case isn't committed?

[luin]

@alecgibson Thanks! Functionality-wise it LGTM. Will consider if there are any opportunities for refactoring.

Is there a reason that test case isn't committed?

It was caught by fuzzing tests, and I just haven't had a chance to commit it.