Add support for syn 2.0.40's new Expr::Group to dsl::auto_type

[Ten0]

This expression may get yielded when parsing $a: expr from other macros.

[weiznich]

The change itself looks good 👍

I would prefer to have a some sort of test somewhere that verifies that the code compiles.

Also I would like to use the possibility to point again to #3783 for the PR that fixes most of the type aliases to work with auto_type.

[Ten0]

There aren't tests for #[dsl::auto_type] besides the doctests so far and that doesn't feel worth putting in the doc, and unfortunately this slipped out of my mind and I can't remember the precise specifics of how to reproduce.
But that did fix my issue in a macro and matches the syn documentation so I'm confident enough that this is correct and we're unlikely to break it in the future.
I may add a test for this in the test file from the other PR but for now I think it would be unreasonable to dedicate that amount of investigation/merge time for such a small thing.

[weiznich]

That sounds ok. I always forget that the #3783 is not merged yet as that PR adds a bunch of "tests".

EDIT: I've rebased this branch to make the failing CI less likely.