Implement syntax enforcment for Pallet<T>(_);

[gui1117]

As a follow up on #8091

This PR enforce the new syntax.
For now nothing is enforced on the fields of pallet.

If this PR doesn't get through before 3.0 I think we can close it.

[bkchr]

Why enforce it?

[gui1117]

I thought some ppl could make typos like:

#[pallet::pallet]
pub struct Pallet(_i);

And would have preferred a more precise error message than just _i is unknown type or something.

But actually I agree it isn't really needed. I let the PR open if some ppl have strong opinion to have it, otherwise I'll close it.

[bkchr]

Ahh. Could you not just allow only _ and if someone tries to use _something you reject it? But, if the people want to use the explicit PhantomData or whatever, let them do that. The compiler will tell them when it is incorrect :D

[shawntabrizi]

I agree. I do not think enforcing this makes sense.

[gui1117]

ok, with the 3.0 released, I think we can wait to see if ppl get confused by syntax error and maybe improve on this like you suggested or maybe it is not needed at all.
I close it then.