Deactivate this extension on non-executable code blocks

[Bengt]

This extension gets activated by fenced code blocks, but it does not deactivate when the code is not actually runnable in a terminal:

This is fine:

```sh
echo "Hello, World!"
```

This is fine:

```bash
echo "Hello, World!"
```

This is fine:

```zsh
echo "Hello, World!"
```

This is fine:

```PowerShell
echo "Hello, World!"
```

This is bad:

```JSON
{"echo": "Hello, World!"}
```

This is bad:

```verbatim
echo "Hello, World!"
```

Please provide some way to deactivate this extension on a case-by-case basis.