[First Draft] Add @gc as a Function Attribute

[jmh530]

On Friday, 14 June 2024 at 16:20:02 UTC, Quirin Schroll wrote:
> Permalink: 
> https://github.com/Bolpat/DIPs/blob/8a26c2545b0a6926d799e9d8dc8c3662e58d516b/DIPs/DIP-4NNN-QFS.md
>
> ---
>
> **Abstract:** Add `@gc` as a new function attribute that acts 
> as the inverse of the `@nogc` attribute.

The DIP makes it seem like it is an escape-hatch for @nogc. But 
then @nogc would no longer be providing any guarantees.