Support syntax highlighted code within backticks

[bramp]

Github markdown allows you to write code within backticks, ie:

```html
<h1>Hello</h1>
```

It would be nice if Hugo supported this as well, instead of having to write:

{{< highlight html >}}
<h1>Hello</h1>
{{< / highlight >}}

This would make my markdown more portable.

[bep]

Duplicates #362

[github-actions]

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