Add editorconfig, so we get similar code styles across editors

.editorconfig

@@ -0,0 +1,18 @@
+# Config for our editors.
+# See: https://EditorConfig.org
+
+# top-most EditorConfig file
+root = true
+
+# Unix-style newlines with a newline ending every file
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+indent_style = space
+indent_size = 4
+
+# Matches package.json
+[package.json]
+indent_size = 2
+