.NET SDK doesn't handle all invalid characters in the root namespace

[marcpopMSFT]

https://github.com/dotnet/sdk/blob/main/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.props#L42

The code already handles spaces but should handle dashes as well as a starting number. For a dash, convert it to an underscore. For a starting numeral, add an underscore so 13-monkeys becomes _13_monkeys