Replace .NET 7 target with .NET 8 rc1

[jasongin]

Fixes #116

I am replacing the .NET 7 target with .NET 8. Version 7 is not an LTS release, while 6 and 8 are, so it makes sense to support .NET versions 472, 6, and 8 for the near future.

.NET 8 is now RC1 status which has a go-live license. Merging this change will require .NET 8 RC1 SDK to develop/build the node-api-dotnet project, but NOT to develop applications that use it. (Any of .NET 472, 6, 7, or 8 runtimes should still work.) Of course, we'll update from RC1 to the released .NET 8 version when it's available in a couple months.

This update required more work than just replacing 7 with 8 in a few places:

• The code for loading .NET HostFxr is updated to allow a prerelease version (at least for now).
• .NET 8 disables dynamic code generation when compiling with PublishAot=true property. For most applications it makes sense to have the managed code behave the same as the Native AOT compiled app. But node-api-dotnet depends on Reflection.Emit in its managed host (not in its native host). To work around that I added a special aot target, with separate output path, that is built and published when packaging (and for tests).
• The source generator assemblies are in a "collectible" assembly load context in .NET 7 and 8, which is incompatible with the default behavior of Expression.Lambda(). I had previously used a workaround for .NET 7 (to reload the generator assemblies as non-collectible) but that doesn't work anymore on .NET 8. This change includes code to explicitly create the lambda delegate types in a collectible context, which resolves the issue. It is a lot more code but is a better solution since now the source-generator can be properly unloaded (and reloaded to pick up changes).
• The VSTestNoBuild behavior changed a bit in the .NET 8 SDK, so I had to update the logic that is used to conditionally skip the dependency builds during the tests.
• The .NET 8 SDK added more analyzer rules checked by dotnet format. So there are small updates in several places to resolve those.

Additionally, I noticed the .NET 6 targeted tests were mistakenly using the .NET 7 or 8 runtime. The build and host logic is updated to fix the issue.

[jasongin]

There's an issue with the source-generator on Max & Linux that causes many of the tests to fail. I know a workaround, but it will be somewhat complicated to implement.

[vmoroz]

LGTM