Add a .editorconfig file

This commit adds an .editorconfig file: it ensures that the coding conventions of DevDocs (https://github.com/freeCodeCamp/devdocs/blob/master/CONTRIBUTING.md#coding-conventions) are applied.

Learn more about the .editorconfig file format: https://editorconfig.org/

.editorconfig

@@ -0,0 +1,8 @@
+root = true
+
+[*]
+charset = utf-8
+indent_style = space
+indent_size = 2
+insert_final_newline = true
+trim_trailing_whitespace = true