elan: Add version 4.0.0

[jack-mil]

Modeled off of the rustup manifest. But uses simple github releases for checkver. Only x86_64 builds available

This manifest will add an environment variable $ELAN_HOME, and modify the $PATH variable to include the persist directory This is because elan is itself just a downloader of other tools (leanc, lake, etc) that need to be available on PATH.

Closes #6725

• [x] Use conventional PR title: <manifest-name[@version]|chore>: <general summary of the pull request>
• [x] I have read the Contributing Guide

[github-actions[bot]]

All changes look good.

Wait for review from human collaborators.

elan

• [x] Description
• [x] License
• [x] Hashes
• [x] Checkver
• [x] Autoupdate