[First Draft] Add @gc as a Function Attribute

[Walter Bright]

On 6/14/2024 9:20 AM, 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.
> 

I've been thinking about a `@default` attribute, which will reset the state of 
attributes to the defaults. This will obviate the need for @gc, @impure, @throw, 
and so forth.