Add snapcraft.yaml

[ChrisMacNaughton]

Snapcraft makes Linux packaging very simple in a cross-distro
way. This adds the snapcraft.yaml file to setup a snap of ripgrep

[ChrisMacNaughton]

I have already registered the snap package name ripgrep, will pass ownership of that over and am happy to help with setting up automated builds!

[Phrohdoh]

Why snapcraft over flatpack?

I know next to nothing about both but there are multiple offerings available and we should probably weight the pros and cons before selecting one (I believe providing both defeats the purpose).

[BurntSushi]

This isn't the place to debate snapcraft vs flatpak.

While I appreciate this PR, I know nothing about snapcraft/flatpak and I don't have any plans to use them in the immediate future, which makes it hard for me to maintain. Is it necessary for snapcraft.yml to be in this repo?

(I did make an exception for Brew since so many people are using it, although even that will eventually go away once SIMD works on stable Rust.)

[ChrisMacNaughton]

I'd rather not discuss one versus the other as well, although a benefit to snaps is that they are available by default on any Ubuntu >= 16.04.

Regarding inclusion in the repository, it is possible to use snaps without including them in the repository but it is a bit easier to deploy into a testing channel with snaps from CI if it's in tree.

[BurntSushi]

All right. I'll merge this for now and we can see how it goes. Thanks!

[ChrisMacNaughton]

@BurntSushi Feel free to ping me (or the snapcraft team) if things are needed to help maintaining it :)