.NET Standard 2.0 support

[zxbmmmmmmmmm]

Description

I would like to add Open Meteo API support to my UWP weather application

Unfortunately, UWP only supports .NET Standard 2.0

Describe the solution you'd like

Change the TargetFramework and LangVersion of the project

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>

  <PropertyGroup>
    <LangVersion>latestMajor</LangVersion>
  </PropertyGroup>

All unit tests can pass