New Color customization option editor.activeBackground

<!-- ⚠️⚠️ 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. --> Would it be possible to add a ```workbench.colorCustomizations ``` that only effects the background color for the active editor. So in addition to having editor.background which is the default editor background we can highlight the active editor with a different color so its easier to see which editor has focus. The tab active color customizations aren't a good solution.