fix: allow include_dir! to be called from within another macro

[dnbln]

Previously, invoking include_dir! from within another macro using a $...:literal matcher argument, include_dir! rejected it because it is wrapped in a TokenTree::Group with Delimiter::None delimiter. This patch allows include_dir! to be used from other macros.

I've also added a test for it.

[dnbln]

Hi @Michael-F-Bryan, could you let me know if you intend to support calling include_dir from another macro? Because right now the only way to work around that is to have a whole other proc macro crate just to unwrap the string literal from the group before giving it to include_dir, so that include_dir can get an ungrouped string literal, which is not great