flexible tab / space handling

I've just realised that tabs are inserted as ... well ... tabs (yuck). I'd love to be able to configure whitespace handling, or at the very least have the option for visible whitespace so that I know when I've mixed tabs and spaces in the same file.

No file, test, or entry point is named. First locate the browser editor's whitespace handling and rendering code, then clarify whether the scope is configurable insertion, visible whitespace, or both; done should include a reproducible check for mixed tabs and spaces.

Written by the indexing model from the issue text.