Packaging improvements

[bording]

While working on #107, I noticed some other things I could do to improve the packages that are produced.

These changes include:

• Added a Directory.Build.props file to be able to set solution-wide properties once.
  • I didn't look through all the project files to see if there was anything pre-existing that made sense to move to this file.
  • Added properties to create symbol packages to aid in debugging, including being able to step into the library code thanks to Source Link in the PDBs.
  • Set ContinuousIntegrationBuild to true when building on CI. This ensures that deterministic builds are enabled.
• Replaced the RepositoryUrl setting in project files with the property that tells the SDK to populate the information instead. (Packaging properties like this would be a good candidate to move into Directory.Build.props)
• Added a global.json file with "rollForward": "latestFeature" set. I noticed that your workflow files had dotnet-version: 9.0.x in them, but it turns out that even with that set, as soon as the GitHub Actions task runners have a newer version, that version will actually get used to build regardless of what the dotnet-version is. Adding a global.json file is only way to ensure the specified range (9.0.x) is always used.
  • To avoid having to worry about mismatched values to maintain in two different places, I also updated the workflow files to instead look at the global.json file for what version to install.

[github-actions]

The documentation preview is available at https://preview.netcord.dev/109.

[KubaZ2]

Thank you!