Add gnome-builder to the list of Rust "first-class editor support" tools

[Unisa-Merni-392]

Summary

Motivation

[senekor]

I think I'm going to close this as "not planned". Gnome Builder is not a general purpose editor and therefore not high up on the popularity list.

The goal shouldn't be to provide an exhaustive list of editors with support, that would be impractical. Rust is a mature language at this point, it would be more surprising to see a serious code editor without Rust support. Still, having a list with the most popular ones doesn't hurt.