Fix extension stripping, add tests, improve artifact module organization

[filiptibell]

In version 0.1.6 of Rokit, more specifically commit 1c6b7ef, Rokit started stripping extensions from file names to fix artifact names such as toolname-win64.zip not correctly being detected as Windows OS.

It turns out however that the stripping mechanism was too greedy, and tool names such as lune-0.8.6-linux-x86_64.zip, with version numbers in them, had 8, 6-linux-x86_64, and zip "extensions" stripped away, leaving no OS or architecture information left for Rokit to parse.

This PR fixes the above issue and adds two safety mechanisms to prevent it from happening again:

• Only known extensions such as "zip", "tar", and others are stripped.
• A maximum of 2 extensions (the maximum amount useful to Rokit) are stripped.

Stricter tests, including real world tool artifacts, have also been added.

---

Business logic for the artifact module has now also been split into three modules to make debugging and reading easier:

• format.rs for the ArtifactFormat enum and its parsing
• provider.rs for the ArtifactProvider enum and its parsing
• util.rs for the new extension splitting mechanism, and its tests