Building in .NET Framework 4

It's great that this library supports .NET Framework 4 – that's what makes it superior to System.CommandLine for our use case. However, we've only managed to use the pre-built DLL. When trying to build the project from master with our MSBuild version, it builds nothing and emits this warning:
```
warning MSB4078: The project file "src\CommandLine\CommandLine.csproj" is not supported by MSBuild and cannot be built.
```
Apparently the format of the .csproj file, with ``, is only supported from MSBuild version 15.0 onwards, while the .NET Framework 4 MSBuild version (4.8 in our case) only supports .csproj files with ``.

Has anyone had success building this library under .NET Framework version 4?

從 src\CommandLine\CommandLine.csproj 開始，使用回報的 .NET Framework MSBuild 4.8 重現 build，並檢查 MSB4078 警告。比較 project 格式與 MSBuild 版本支援的格式；完成的標準是 library 能夠在 .NET Framework 4 下從 master 建置，而不是只能依賴預先建置的 DLL。

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