Add .editorconfig to repo.

This will enforce some editors to respect format of some files.

.editorconfig

@@ -0,0 +1,15 @@
+root = true
+
+[*.{ts,json}]
+charset = utf-8
+trim_trailing_whitespace = true
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = false
+
+[package{,-lock}.json]
+insert_final_newline = true
+
+[tsconfig.json]
+indent_size = 4