Add editorconfig support for Postgres spec files

I was reading through some spec files today, and my editor was resolving 
the tab width as 8. I looked at some of the spec files, and we seem to 
have standardized on a tab width of 4 with tabs expanded into spaces. 
I decided to encode this in the .editorconfig file for convenience.

PS: I am working on a tree-sitter grammar for these files. I'll probably 
try to set up a TextMate grammar was well for those who use editors that 
support that. I'll share that later.

-- 
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)