Support .NET Framework 4.6

[rdeago]

Background and motivation

We need to support .NET Framework 4.6 to close Tenacom/ReSharper.ExportAnnotations#20 but it's a no-go while this library only supports .NET Standard 2.0.

We also need to close that issue to solve some of the problems we're having in #11 so we'd better do it by yesterday.

Proposed enhancement

Support .NET Framework 4.6.

Implementation proposals

Add net46 to target frameworks.

Usage examples

Given a .csproj file containing the following:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>net46;netcoreapp3.1</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Cecil.XmlDocNames" Version="{NEXT_VERSION}"/>
  </ItemGroup>

</Project>

where {NEXT_VERSION} is a version where this issue has been closed, it should build correctly for both TFMs.

Risks

The net46 TFM could require us to downgrade the code to C# 7.3, thus losing nullable reference types. In that case, we can use ReSharper to check for nullability issues; how to perform the necessary checks in CI remains to be researched.