Add .editorconfig

The main use right now is getting properly spaced diff views on
GitHub, but perhaps this will also help developers with editors that
we currently don't have setup recipes for.

The settings mirror mostly what's currently in .dir-locals.el.

Discussion: https://www.postgresql.org/message-id/flat/273cb788-bbb2-ff34-ad6f-5192b44e5049%402ndquadrant.com

.editorconfig

+root = true
+
+[*.{c,h,l,y,pl,pm}]
+indent_style = tab
+indent_size = tab
+tab_width = 4
+
+[*.{sgml,xml}]
+indent_style = space
+indent_size = 1
+
+[*.xsl]
+indent_style = space
+indent_size = 2