Shift up in first text block should select to 1st character

[jasmussen]

There's an issue with shift selecting. GIF:

Shift select across paragraphs works.

Setting the caret in the last text block and shift selecting downwards, selects until the end of the text in that block, as it should.

Setting the caret in the first block and shift selecting upwards, doesn't seem to select until the beginning of the text in that block, as would be expected.

This was a split out from #5548 (comment)

[designsimply]

Tested and found that setting the caret in the first block and selecting upward does not select until the beginning of the text in that block and setting the caret in the last block and selecting downward does not select until the end of the text in that block.

Video: 56s
^{Seen at http://alittletestblog.com/wp-admin/post.php?post=13999&action=edit running WordPress 4.9.7 and Gutenberg 3.3.0 using Firefox 61.0.1 on macOS 10.13.6.}

[youknowriad]

Just to confirm @aduth you were looking at this recently right? Is there a PR to link to?

[aduth]

Yes, see #13638 .