[First Draft] Add @gc as a Function Attribute

[Quirin Schroll]

Permalink: 
https://github.com/Bolpat/DIPs/blob/8a26c2545b0a6926d799e9d8dc8c3662e58d516b/DIPs/DIP-4NNN-QFS.md

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**Abstract:** Add `@gc` as a new function attribute that acts as 
the inverse of the `@nogc` attribute.