Form email body with Twig include and 'with' clause, variables not available in included template

I'm trying to create an email body like:

```yaml
body: "Thank you for your message via my website. Here is a copy for your records:\n\n{% include 'forms/data.txt.twig' with {'hide_fields': ['ip', 'timestamp']} %}"
```
(I want to customise the formdata template to hide the fields named, that's kind of irrelevant except it shows a use case.)

I created theme copies from the form plugin `themes//templates/formdata.html.twig` and `themes//templates/forms/default/data.html.twig`.

It looks like my variable `hide_fields` is not being passed through and I tested this by adding `{{ dump(hide_fields ) }}` to `formdata.html.twig` and it dumped `null`.

I traced this back some way but eventually got lost. It looks like the email plugin's `Email->buildMessage()` calls `\Grav\Common\Twig\Twig->processString()`. When I dump the result there with PHP, it has been lost and I can't quite figure out where. My PHP debugging toolkit isn't that sharp. Just raising the possibility this is an issue in the email plugin or Grav core. Hope it helps.

Should my Twig expression in the email body be supported? I have a feeling I've even done this in the past, though I couldn't find it.

Reproduce the email body from the issue using the theme copies at themes//templates/formdata.html.twig and themes//templates/forms/default/data.html.twig, including the dump of hide_fields. Trace Email->buildMessage() into \Grav\Common\Twig\Twig->processString() and identify where the include context is lost. Done means determining whether the Twig expression is supported and preserving hide_fields in the included template if it is.

Written by the indexing model from the issue text.