Twigilless attribute specifications should be forbidden

[lizmat]

From rakudo/rakudo#6097:

role A {
    has %types;
    %types{::?CLASS.^name} = ::?CLASS;
    method dump-types {
        dd %types
    }
}

===SORRY!=== Error while compiling foo
Variable %!types used where no 'self' is available
at foo:3
------>     %types⏏{::?CLASS.^name} = ::?CLASS;

I think that the use of has $foo without a sigil should be forbidden exactly because the expectations that it brings, and the puzzling error messages that it may create.

[librasteve]

I 100% support this - I was very surprised to learn twigilless has attributes existed in the first place and I think that they cause confusion and are error prone.

[ab5tract]

I also completely support this. My only regret is that we didn't decide this before adding it to RakuAST.

EDIT: To be clear, I express this regret simply because of the effort it cost to do so, not because of any "we can't go back now, it's already done!" kind of sentiment.