Rust TokenStream explorer

Hey there, love the project!

I see Rust support has now been added in the form of `syn`, which is fantastic. However would it be possible to add another parser for `proc_macro2` that just dumps the tokens it parses? It'd be really handy for developing procedural macros.

I was looking into the source to see if I could figure it out on my own but I'm not entirely sure how the project is set up - I see there's `astexplorer-syn` but I'm not sure where that's coming from.

Thanks!