Sets the tab size to 4 in the Markdown preview for code blocks in Markdown documents. I consider a tab size of 4 being standard and the default of 8 being unbearable. The few people, if any, using a different tab size than 4 can use the custom markdown preview style feature, if it is going to be fixed anytime (https://github.com/microsoft/vscode/issues/77290).
[`tab-size` is still marked *experimental* in the MDN](https://developer.mozilla.org/en-US/docs/Web/CSS/tab-size), but it doesn't hurt, if not supported. However, in the used Chromium version it is supported.
