feat: add ferris as alt rust icon

[tom22k]

Description

Added Ferris, the "unofficial mascot for Rust", as an alternative Rust icon.
Used the flat SVG from rustacean.net, optimised it and replaced the colours with Material Design colours.

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[lucas-labs]

I use a simplified version (for better rendering at 16x16 px) of ferris for special files (lib.rs, mod.rs, etc), with different colors:

Maybe instead of adding it as unassigned, that might be a better use for our friend ferris. Although I don't know what other rust devs would think about it.

Note

Here's the svg source, btw:
https://github.com/lucodear/lucodear-icons/blob/main/icons-lc/rust/file-rust-ferris.svg

And I have several icons for "special" files for rust in this folder here too, althogh not all of them are "good":
https://github.com/lucodear/lucodear-icons/blob/main/icons-lc/rust