Add support in getTextChangeMutation() for removed text nodes

[taylorhadden]

There are two common cases where mutation parsing gives up and falls back to a whole-document diff:

1. When a delete replaced a text node with a <br>.
2. When a delete removed a text node from within a line.

In both cases, getChangedLineRange() doesn't resolve to a correct range as it early-outs on the deleted text element. This results in diffing the entire document.

getTextChangeMutation() already handled replacing a <br> with text, so I modified it to also handle the reverse. I also added the handling of a removed text node that had a previous sibling. Pulling the index resolution into the function allowed these slightly different situations to be handled appropriately.

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If you feel this change modifies getTextChangeMutation() too much, I can instead look to improve getChangedLinkRange() so that these situations fall back to a line diff instead of a whole-document diff.