Allow to configure environment variables for extension host

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> I've got a really simple problem: often I open VS Code from a terminal, because I need to run `export PATH=...` in order to be able to use some extension inside of VS Code that uses a binary to work, e.g., [this Lilypond extension](https://marketplace.visualstudio.com/items?itemName=lhl2617.VSLilyPond), which works really well (thank you @lhl2617!), uses the `lilypond` external binary. Perhaps a workspace could have environment variables in addition to project folders? :thinking: