Support for email templates in Kirby query language

[lukasbestle]

Because PHP templates have the problem of swallowing newlines (see getkirby/kirby#2750), email templates in the Kirby query language (Str::template()) would be more robust.

PHP templates should still be the default to ensure backwards-compatibility, but there should be a way to optionally use the query language. Possible options:

• Introduce a new file extension for our Str::template() syntax so that templates in the query language would be autodetected.
• Make this a prop for $kirby->email() that allows to define the template type explicitly (in this case the templates would still use the .php file extension).