Introduce .editorconfig file with basic code formatting rules

Many IDEs (such as IntelliJ IDEA) and editors already use the .editorconfig file if available.
For most popular editors there’s a plugin that adds EditorConfig support.

See https://editorconfig.org/ for more details.

.editorconfig

@@ -0,0 +1,9 @@
+root = true
+
+[*]
+insert_final_newline = true
+
+[*.java]
+indent_size = 4
+indent_style = space
+trim_trailing_whitespace = true