Previously, only Text and Image could be nested within Text. Now, any
view can be nested within Text. One restriction of this feature is
that developers must give inline views a width and a height via
the style prop.

Previously, inline Images were supported by using iOS's built-in support
for rendering images with an NSAttributedString via NSTextAttachment.
However, NSAttributedString doesn't support rendering arbitrary views.

This change adds support for nesting views within Text by creating one
NSTextAttachment per inline view. The NSTextAttachments act as placeholders.
They are set to be the size of the corresponding view. After the text is
laid out, we query the text system to find out where it has positioned each
NSTextAttachment. We then position the views to be at those locations.

This commit also contains a change in RCTShadowText.m
_setParagraphStyleOnAttributedString:heightOfTallestSubview:. It now only sets
lineHeight, textAlign, and writingDirection when they've actually changed. This
avoids unnecessarily dirtying the shadow tree which is important in some inline
view scenarios.

The vertical alignment should now be consistent
with how iOS vertically positions an inline
image rendered with an NSTextAttachment.