This function failed to account for the case where the node was
contained by, but not equal to, the range's startContainer.

In new browsers we could just use `Range.isPointInRange()`. Since that
is not available in IE 10/11, add an implementation adapted from
Blink's `Range::isPointInRange()` C++ impl, minus support for the
`offset` parameter.

This fixes an issue where the adder could fail to show up when making
certain selections because `getTextBoundingBoxes()` could incorrectly
fail to find the text nodes within certan ranges.

Fixes #3521