Backspace does not work before a folded block

[alexeyinkin]

Use this code:

import argparse
import os

def fn(argv=None):
  pass

Fold the imports. Navigate to the 2nd blank line. Press backspace.

Expected: The line is deleted. Actual: Cursor moves one line up.

[alexeyinkin]

• Done in https://github.com/akvelon/flutter-code-editor/pull/249