Side-by-side without fullscreen

**Describe the bug**
If the editor is configured to have a side-by-side toolbar button but not the fullscreen button then pressing the button gets the user stuck in fullscreen mode.

**To Reproduce**
Steps to reproduce the behavior:
1. Configure editor with the side-by-side toolbar but without the fullscreen button.
2. Click on the Side-by-side toolbar button.
3. You're now stuck!

**Expected behavior**
The side-by-side button should not put the editor into fullscreen mode, especially not if the fullscreen button is not present in the toolbar.

**Version information**
- EasyMDE version: 2.16.1

**Additional context**
This can be set by:
```javascript
const easyMDE = new EasyMDE({ sideBySideFullscreen: false });
```
Which I think:

1. Should be the default setting.
2. Or be autodetected if the fullscreen button is not in the toolbar.

Start by reproducing the issue with side-by-side enabled, the fullscreen toolbar button omitted, and sideBySideFullscreen set to false. Trace the editor's side-by-side and fullscreen toolbar behavior, then verify that clicking Side-by-side does not enter fullscreen or leave the editor stuck in that mode.

Written by the indexing model from the issue text.