Update .editorconfig to what we have in practice (#1545)
While editing the impl guide markdowns I tried to be inline with what seemingly more common way to indent them: spaces. However, despite that I changed it kept reseting. Turned out the culprit is the .editorconfig file. This commit addresses this issue. I didn't try to deduplicate the rules since I found that the formal specification is a bit ambigious and it is not a big deal anyway.