Zero-width space in code block fences

[Lakitna]

• [x] Searched existing issues to avoid creating duplicates.

• [x] Confirmed that it can be reproduced in built-in themes without customized css.

  If it only exists in 3rd party themes or css, you can still report it, but please attach the theme target or the css file. We may not "fix" it, if it is caused by 3rd party themes or css styles that we cannot support.

• [x] Searched http://support.typora.io/

---

Describe the bug

Typora inserts zero-width spaces in nested code block fences. This breaks my Markdown tooling downstream. Zero-width spaces are represented by red dots in this image:

To Reproduce

Create a nested code block outside of source code mode. That's it.

Expected behavior

Typora creates the following source code:

````
```
hello
```
````

This is proper Markdown in all tooling I've used so far, including here on Github :) Though Typora is not able to render it properly:

Alternative solution:

Typora creates the following source code:

~~~

hello

~~~

This does render as expected:

Sample Markdown File

As above

Desktop (please complete the following information):

• OS: Windows 10 Pro 19042.1052

Typora Version 0.10.11 (beta)

Additional context

Might be related to #403

[abnerlee]

3110

[abnerlee]

should be fixed

[Lakitna]

Confirmed that it's fixed :) Thank you!