Improve arch and system autodetection

[gdevenyi]

The autodetection works in other cases, but fails in others, example:

my system is linux and amd64

moar works: https://github.com/walles/moar/releases/tag/v1.27.2
named: linux-amd64

micro does not: https://github.com/zyedidia/micro/releases/tag/v2.0.14
names: linux64, linux64-static

lsd does not: https://github.com/lsd-rs/lsd/releases/tag/v1.1.5
names: x86_64-unknown-linux-gnu, x86_64-unknown-linux-musl

I'm not necessarily saying that in these failing cases that stew should absolutely find the right archive (micro is doing it wrong, but lsd is using a standard naming scheme), but it should at least provide me with a partially filtered list for me to choose from.

[gdevenyi]

stew doesn't properly detect stew releases either :)

$ stew search stew
* Choose a GitHub project: marwanhawari/stew [⭐️179] 🥘 An independent package manager for compiled binaries.
marwanhawari/stew
! Could not automatically detect the release asset matching your OS/Arch. Please select it manually:  [Use arrows to move, type to filter]
> checksums.txt
  stew-v0.4.0-darwin-amd64.tar.gz
  stew-v0.4.0-darwin-arm64.tar.gz
  stew-v0.4.0-linux-386.tar.gz
  stew-v0.4.0-linux-amd64.tar.gz
  stew-v0.4.0-linux-arm64.tar.gz
  stew-v0.4.0-windows-386.tar.gz