feat(linux.rs): Add support for Funtoo

[Lazerbeak12345]

What does this PR do

Adds support for updating Funtoo Linux systems.

Standards checklist

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself (I tested it on Funtoo linux ONLY. I'd advise testing it on gentoo as well)

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

[SteveLauC]

HI, thanks for the PR!

I wonder can we have an "os-release" file there, and also a unit test like this :)

[Lazerbeak12345]

Got those pretty easy.

[SteveLauC]

Thanks!