[Bug] Trying to merge two blocks that don't have a `merge` method

[neSpecc]

In 2.18 release there was a mistake with adding a public access modifier to the block.mergeable getter.

And now, it is a public method instead of a getter. As a result, trying to press Backspace between two same blocks (that don't have a merge method) occurs an error:

Unhandled Promise Rejection: TypeError: this.tool.merge is not a function. (In 'this.tool.merge(data)', 'this.tool.merge' is undefined)

Steps to reproduce:

1. Go to example.html
2. Make to List blocks
3. Place the caret at the beginning of the second List
4. Press Backspace
5. You'll see an error

Expected behavior:

Error is not thrown.

Editor.js version:

2.19

[robonetphy]

@neSpecc
I replace public with get it working perfectly fine on the paragraph plugin but not for list but there is no error on the console.
is that correct result?

[neSpecc]

@neSpecc
I replace public with get it working perfectly fine on the paragraph plugin but not for list but there is no error on the console.
is that correct result?

open a draft pr please, and attach it