tpl: Narrow down the usage of plain text shortcodes when rendering HTML

tpl: Narrow down the usage of plain text shortcodes when rendering HTML

After this commit, if you want to resolve `layouts/_shortcodes/myshortcode.txt` when rendering HTML content, you need to use the `{{%` shortcode delimiter:

```
{{% myshortcode %}}
```

This should be what people would do anyway, but we have also as part of this improved the error message to inform about what needs to be done.

Note that this is not relevant for partials.

Fixes #13698