Explcit impure function attribute

[Richard (Rikki) Andrew Cattermole]

On 25/06/2024 4:38 AM, Quirin Schroll wrote:
> Make |@impure| a compiler-recognized AtAttribute. This has the downside 
> of being asymmetric: |pure| has no |@|, but its inverse |@impure| would 
> have it.

So add it.

Deprecate, then remove without ``@`` in newer editions.

Making all attributes consistent with using ``@`` is a good thing.