Introduce .editorconfig

Summary:
By default, GitHub renders tabs as 8-spaces-width, which screws up postgres-inherited formatting.
However, GitHub respects `.editorconfig` file, so here we are.

Test Plan:
Manual inspection of file styles in GitHub

Jenkins: skip

Reviewers: bogdan, mikhail

Reviewed By: mikhail

Subscribers: bogdan, mikhail

Differential Revision: https://phabricator.dev.yugabyte.com/D8600

.editorconfig

@@ -0,0 +1,22 @@
+# https://EditorConfig.org
+
+root = true
+
+[*]
+charset = utf-8
+max_line_length = 100
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 2
+tab_width = 4
+trim_trailing_whitespace = true
+
+# Indentation override for all postgres-style code
+[src/{postgres/**.{c,h,mk,in},**/{Makefile,configure}}]
+max_line_length = off
+indent_style = tab
+indent_size = tab
+
+[*.md]
+trim_trailing_whitespace = false