Not possible to line break inside "[\]" shortcut with vim mode enabled

### Is this a hackmd.io issue?

- [X] Yes, the bug happens on https://hackmd.io

### What's the problem
It's very hard for a vim user to line break inside "[\\]" after "[\\",
It always popup a create note shortcut and press ESC did close the popup but also leave edit mode.
Therefore it's not possible to easily line break without doing some trick like copy/past or remove ] or press [ESC, A, ESC, I, Enter],
which is a lot of keyboard action for a simple line break.

#### Current behaviour


Provide steps to reproduce:

1. Open a new note with
```
[\]
```
2. change to vim mode
3. press the left arrow key
4. see the error

#### Expected behaviour

1. An option to disable the [\\] shortcut feature.
2. ESC close pop up first but it won't leave vim edit mode.

#### Environment

I use HackMD on:

**Desktop**
- OS: Ubuntu, Windows
- Browser: chrome
- Browser Version: [89]