Move shared/common MSBuild and package properties into Directory.Build.props

[samatstariongroup]

Context

Directory.Build.props currently only centralizes a handful of properties (Company, Copyright, RepositoryUrl, RepositoryType, Deterministic, NeutralLanguage, SatelliteResourceLanguages).

However, ReqIFSharp/ReqIFSharp.csproj and ReqIFSharp.Extensions/ReqIFSharp.Extensions.csproj still duplicate a number of properties that are identical (or nearly identical) across both packages:

• LangVersion (12.0 in both)
• PackageProjectUrl (https://reqifsharp.org in both)
• PackageLicenseExpression (Apache-2.0 in both)
• PackageTags (ReqIF mbse modeltopia in both)
• GeneratePackageOnBuild (False in both)
• RequireLicenseAcceptance (false in both)
• IncludeSource / IncludeSymbols (true in both)
• GenerateDocumentationFile (true in both)
• GenerateSBOM (true in both)
• PackageReadmeFile (README.md in both)
• The <None Include="..\README.md" .../>, <None Include="..\LICENSE" .../>, <None Include="..\NOTICE" .../> pack-file item group (identical in both)
• The Microsoft.Sbom.Targets PackageReference (present, identically configured, in both)

Duplicating these across csproj files means every package-wide change (e.g. bumping LangVersion, changing the SBOM/packaging policy) has to be applied twice and is easy to let drift.

Proposal

Move the properties/items above that are genuinely shared into Directory.Build.props, and optionally split out a Directory.Build.targets or a dedicated Directory.Packages.props-style packaging props file (e.g. Directory.Package.Common.props) imported from each packable project, so that:

• ReqIFSharp.csproj and ReqIFSharp.Extensions.csproj only retain what's genuinely per-package: TargetFramework, Version, Title, Description, PackageId, PackageIcon, Authors, PackageReleaseNotes, and the icon None item.
• The Microsoft.Sbom.Targets PackageReference and the README/LICENSE/NOTICE pack items are declared once and apply to any packable project (test/non-packable projects should not pick them up — e.g. guard with a condition on IsPackable).

Notes

• Test projects (ReqIFSharp.Tests, ReqIFSharp.Extensions.Tests) target net10.0 and are not packed, so packaging-related props should not leak into them — use an IsPackable/$(MSBuildProjectName) condition or a separate props file imported only by the two library projects.
• LangVersion is a good Directory.Build.props candidate since it's already documented as pinned to 12.0 repo-wide in CLAUDE.md.