Fix ASTextNode truncated size calculation ignoring attributes in the last line

[samhsiung]

Fixes #930

ASTextNode uses ASTextKitTailTruncater to calculate the size of the rendered truncated text by replacing the last line with the _composedTruncationString if the text is long enough to truncate.

However _composedTruncationString doesn't have attributes from _attributedString at the time of size calculation so attributes such as NSParagraphStyle were being omitted resulting in an incorrect size.

We fix this by syncing attributes of _composedTruncationString with _attributedString whenever _attributedString is updated in ASTextNode.

We also updated ASTextNodeTests to test for different line spacing applied throughout text.

[appleguy]

@samhsiung thank you for the clear, specific description. It was very important for me to understand this diff. I also particularly appreciate you adding the test.

The attention to detail in our text rendering system is very valuable, and is an expertise that a fairly small subset of engineers have. I'm starting to look at this code more closely myself, due mainly to your lead, and am finding some sweet optimizations - one will go up soon.