storybook: addon-docs: ${...} in code block is executed

Describe the bug In a code block in MDX, inserting ${var} is executed(-ish) and replaced by something that’s not useful. There doesn’t appear to be a way to escape it, and it shouldn’t be neccesary anyway. This worked fine in Storybook 5.2.x but this bug may have creeped in at a later time.

This example MDX snippet:

```jsx
const CustomLabel = styled(Label)`
  ${display}
`;
```

Will be displayed as:

const CustomLabel = styled(Label)`
  ___CSS_0___
`;

I’ve tried to escape it with \${display}, $\{display}, ${\display}. I’ve tried escaping the backticks with `` and \`. None of these escapings fix the problem. Either the escapes are displayed literally (and the bogus ___CSS_0___ remains), or I get an error that says display is not defined. Strangely, even without this error, it still manages to replace ${display} with something.

To Reproduce Steps to reproduce the behavior:

1. Have an MDX with the above code
2. Run it in your Storybook instance

Expected behavior It shouldn’t try to replace anything. It’s not code, it’s documentation.

System: Results of npx -p @storybook/cli@next sb info:

  System:
    OS: Windows 10 10.0.18363
    CPU: (12) x64 Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
  Binaries:
    Node: 10.16.3 - C:\Program Files\nodejs\node.EXE
    npm: 6.9.0 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: 44.18362.449.0

Too bad this command doesn’t output my Storybook version, which is 5.3.14, and for addon-docs as well.

Additional context One workaround is to replace ```jsx with ```, which changes the syntax highlighting. It also stops Storybook from replacing ${display} with a bogus string. Really, the tag after ``` should only specify syntax highlighting and shouldn’t apply any arbitrary parsing or additional code analysis. It should be treated as plain text, and never anything more.