feat(zig-0.15): clean up build process and update to Zig 0.15

[hamptokr]

Changes

• Adds support for Zig 0.15
• Updates to libssh2 1.11.1
• Fixes Windows build
• Updates ci to the latest used in other repos
• Cleans up the build process
• Adding additional options during the build
• Adds docs
• Adds MIT LICENSE so others can use it

This is mostly bringing in my previously external repo: https://github.com/hamptokr/zig-libssh2