fix(block-events): caret losing after backspace after nested list

[neSpecc]

Problem

Pressing backspace at the start of a paragraph after a Nested List causes the caret to be lost.

caret-bug.mov

Cause

There are two problems:

1. The backspace handler incorrectly shows true for areBlockMergeable(). This was due to confused arguments: the first argument is the block to merge into, and the second is the block to merge from. After fixing this, areBlockMergeable() will return false.

2. The Nested List tool has a conversionConfig but lacks a merge() method, making it not mergeable. This case was not covered by tests, so I've added the corresponding test case.

3. During debugging, I noticed that creating/restoring the Shadow caret did not work for Nested Lists. Previously, it added a Shadow caret to pluginContent, which is complex for Nested Lists, and it was added to a wrong place.

Solution

1. Fixed the order of arguments for areBlockMergeable().
2. Changed the argument of createShadow() from pluginContent to lastInput, so the shadow caret is now set properly. Although with the fix №1 this logic won't be called, but I've fixed it anyway.

caret-bug-fixed.mov

And then I'm gonna update the Nested List tool adding a merge() method in there. After that the merging will start working instead of caret moving.