CodePane size not customizable

### Describe Your Environment

What version of Spectacle are you using? (can be found by running `npm list spectacle`)
spectacle@8.3.0
What version of React are you using? (can be found by running `npm list react`)
react@17.0.2
What browser are you using?
Firefox
What machine are you on?
Windows

### Describe the Problem

I made a layout for my presentations that does not use the whole width of the page, by displaying several areas with different background values.
The slide is defined like this:
```





Header


Sub Header




{`
{
"some": value
}
`}




```

Using that slide results in the codepane being wider than the box it is contained in.
I also have no way to define its width other than setting the width in the theme.
Setting the width and height value of the theme to `100%` fixes the issue for me, but i still can't customize the code panes size directly.
I think that customizing the size might be useful, especially in a situation where the code pane is contained in a sub layout

**Expected behavior:** [What you expect to happen]

There is a way to customize the code panes size

**Actual behavior:** [What actually happens]

The code pane size is not customizable which can result in it being too wide