Set LTR on code blocks in edit mode, even when direction is RTL


**Is your feature request related to a problem? Please describe.**
When `direction` is `rtl`, code block lines in edit view are also RTL.

**Describe the solution you'd like**
All code blocks (fenced or indented) should be LTR by default while editing.

**Describe alternatives you've considered**
Perhaps it should be controllable by an option or a button.

**Additional context**
I would note that preview suffers from the same problem. I added a bit of CSS to fix:

``` css
.editor-preview pre {
direction: ltr;
}
```

but there doesn't seem to be a specific CSS class added to code block lines in edit view.