feat: add versioned package installation and caching

[httphypixelnet]

Summary

Add version-aware package installation for packages hosted on GitHub. Package
references can now specify a Semver constraint, and Vale resolves the
latest matching GitHub release before installing it.

Add an on-disk cache for GitHub release metadata and downloaded release
archives. Cache locations follow platform conventions and can be overridden
with VALE_CACHE.

Details

• Resolve Packages entries such as Google@0.7.0 and version ranges against
  GitHub releases.
• Prefer matching release assets, then fall back to the release zipball.
• Cache release metadata with ETags and cache downloaded release archives.
• Add package end-to-end coverage for exact versions, version ranges, invalid
  versions, and cache output.
• Modify E2E suite to work with the updated methods

[jdkato]

Thanks for the work here, but I think this is out of scope for Vale.

For pinning, this is already supported: give a release URL in place of the name.

Packages = https://github.com/errata-ai/Google/releases/download/v0.7.0/Google.zip

That works for any host, and BasedOnStyles reads the same either way (docs).

What's left beyond that is resolving version ranges, and that's where I'd rather Vale not go:

• Ranges need a semver resolver, a GitHub API client, and a cache to keep its traffic tolerable -- GitHub-only, where Packages is host-agnostic today.
• They'd also need a lockfile to be worth trusting. Without one, @^0.7 re-resolves on every sync, so it floats with a ceiling rather than pinning -- not what users expect.

The middle ground you're after is a real want. It's just the expensive end, and I'd rather keep Vale at the two ends than maintain a package manager to reach the middle.