Add documentation of environment variables

[ranma42]

There seems to be no documentation of the environment variables understood/handled by vstest.

For example, VSTEST_TESTHOST_SHUTDOWN_TIMEOUT seems to only appear in the code and a couple of time in issues in this repo (even taking into account a web search).

It would be very convenient to have some document that collects all of the variables and explains them, just like https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-environment-variables

(this is not a bug, but a feature request)