Add GitHub Actions build workflow

Builds Essentials on windows-latest. Acquires the SE references via
steamcmd (app 298740, anonymous) and the Torch references from the
torchapi build server, mirroring what the Jenkinsfile did, with both
cached on a weekly key and fetched by parallel jobs.

Versioning stays with Versioning/version.ps1 (git describe + build
number, with BUILD_NUMBER mapped to github.run_number), which also
writes Versioning/AssemblyVersion.cs before the compile. The ${VERSION}
placeholder in manifest.xml is substituted in the workspace only; the
committed manifest is untouched.

Packages Essentials.* plus the manifest, as the Jenkinsfile did, and
uploads it as an artifact named after the plugin and version.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: Bishbash777