Use & enforce snake_case naming convention on config.json settings (#8062)

* Document and support the established naming convention for config opts

This change:
* Rename `ConfigOptions` to `IConfigOptions` to match code convention/style, plus move it to a dedicated file
* Update comments and surrounding documentation
* Define every single documented option (from element-web's config.md)
* Enable a linter to enforce the convention
* Invent a translation layer for a different change to use
* No attempt to fix build errors from doing this (at this stage)

* Add demo of lint rule in action

* Fix all obvious instances of SdkConfig case conflicts

* Fix tests to use SdkConfig directly

* Add docs to make unset() calling safer

* Appease the linter

* Update documentation to match snake_case_config

* Fix more instances of square brackets off SdkConfig

src/components/structures/RoomDirectory.tsx

@@ -103,7 +103,7 @@ export default class RoomDirectory extends React.Component<IProps, IState> {
 
                 let roomServer = myHomeserver;
                 if (
-                    SdkConfig.get().roomDirectory?.servers?.includes(lsRoomServer) ||
+                    SdkConfig.getObject("room_directory")?.get("servers")?.includes(lsRoomServer) ||
                     SettingsStore.getValue("room_directory_servers")?.includes(lsRoomServer)
                 ) {
                     roomServer = lsRoomServer;