feat: mark GitHub releases as prereleases for prerelease versions

[kieronlanning]

Summary

Creates the GitHub release as a prerelease when the package version is a prerelease (for example 2.0.0-prerelease.25), so prerelease builds are no longer presented as the latest stable release.

Changes

• ReleaseSettings: new MarkPrerelease setting (default rue) plus ShouldMarkPrerelease(NuGetVersion).
• CreateGitHubReleaseModule: passes Prerelease = to the release creation and logs the outcome.
• �ppsettings.json: documents the new Release:MarkPrerelease default.
• Docs: added the setting to the configuration reference and described the behaviour in the pipeline-modules page.
• Tests: ReleaseSettingsTests covers prerelease version + defaults, stable version, and MarkPrerelease = false.

Why

Consumers publish -prerelease.* versions to NuGet, but the GitHub release was always created as a stable release, so every prerelease appeared as Latest. Release metadata is owned by the tool, so the behaviour belongs here rather than in consumer workflows.

Validation

• dotnet build src/Build.slnx -c Release --warnaserror -> 0 warnings, 0 errors.
• dotnet csharpier check . -> clean.
• dotnet test src/tests/Build.IntegrationTests -> 58 passed, 0 failed.

Rollout note

Consumers install the latest stable Purview.Build from nuget.org, so this takes effect for consumer pipelines once a new tool version is published.