`renderContent` Universal Filter in Render Plugin

```jinja2
---
hi: "{% test %}"
---
{{ hi | renderContent("njk") }}
```

Related to #3369

This is different from:

```jinja2
---
hi: "{% test %}"
---
{% renderTemplate %}{{ hi }}{% endrenderTemplate %}
```

which renders plain raw text `{% test %}` because only the content `{{ hi }}` is rendered by the `renderTemplate` shortcode (not the return content inside the variable)