Assembly isolation opt-in at dotnet-script level

[hrumhurum]

Due to difficulties and relative immaturity of AssemblyLoadContext support, assembly isolation should be opt-in at dotnet-script level for now.

This PR implements this by introducing --isolated-load-context command-line option in dotnet-script.

[seesharper]

@hrumhurum The build failed because of some network issue. I can't find where to restart the build. Can you just push an empty commit to restart?

git commit --allow-empty -m "Trigger Build"

[seesharper]

Looks good, thanks 👍

[filipw]

thank you

[seesharper]

@hrumhurum Just wanted to say thanks for reacting so quickly to the problem. 👍😊