editorconfig: add one matching current style

Add a editorconfig (https://editorconfig.org/) config file so most
editors can, by default, get the right indent settings.

.editorconfig

@@ -0,0 +1,19 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8
+
+[*.py]
+indent_style = space
+indent_size = 4
+
+[*.json]
+indent_style = space
+indent_size = 2
+
+[*.{yml,yaml}]
+indent_style = space
+indent_size = 2