Port changes from 3.19 releases to V4

[CharliePoole]

• Introduction of Microsoft.Extensions.DependencyModel causes reflection crashes #1324
• Replace 'dev' label used for development builds with 'alpha' #1545
• Readme doesn't list the .NET Core runner #1508
• The --teamcity option silently fails if the extension is not installed #1507
• Use a programmed search to locate extensions in place of default addins files. #1504
• Simplify locating of addins #488
• Broken Extensions discovery when installed with Chocolatey #1614

[CharliePoole]

Reopening to include fixes from versions 3.19.1 and 3.19.2

[CharliePoole]

This issue has been resolved in version 4.0.0-beta.1

The release is available on:
GitHub.
NuGet packages are also available NuGet.org and
Chocolatey Packages may be found at Chocolatey.org