Update .editorconfig to what we have in practice

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.