Add EditorConfig file (#1930)

* 🔧 add editorconfig

This makes supported editors automatically change their settings to match desired code styles

* 🔧 add extension recommendation for VSCode

This will cause VS Code to prompt the user to install the EditorConfig extension when they open the project.

If this is felt to be too opinionated, I can revert it.

.vscode/extensions.json

@@ -0,0 +1,6 @@
+// Suggested extensions
+{
+  "recommendations": [
+    "EditorConfig.EditorConfig"
+  ]
+}