Wrap source code in code tags

Referencing [this issue on markdown-colorcode](https://github.com/wbaldoumas/markdown-colorcode/issues/83), this package overwrites some of the [behavior used in Markdig](https://github.com/xoofx/markdig/blob/203cfd6508d4f8fca510edd2f66f229ada2c25ad/src/Markdig.Benchmarks/spec.md#fenced-code-blocks) that appears to be in line with [the CommonMark spec](https://spec.commonmark.org/0.30/#example-142), that is, this package does not wrap code using a `code` element. Additionally, if a language is present, the spec specifies a `class="language-[lang]"` attribute as well.

I have not started a fork because I am not sure whether this would be a breaking change or not, however, the general change seems rather simple, adding the following methods to `HtmlClassFormatter` and then wrapping the inner text in `WriteHeader` and `WriteFooter`.

```csharp
private void WriteHeaderCodeStart(ILanguage language)
{
WriteElementStart("code", language.CssClassName);
}

private void WriteHeaderCodeEnd()
{
WriteElementEnd("code");
}
```

In my case, this would make it easier to enforce consistent styling across my app.