Add .editorconfig (swiftlang#78087)

This sets indentation following established precedent. The file itself is identical to the one used by the `swift-syntax` repository.

Author: MaxDesiatov

.editorconfig

@@ -0,0 +1,9 @@
+# editorconfig.org
+
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true
+insert_final_newline = true