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/views/auth/LanguageSelector.tsx

@@ -35,7 +35,7 @@ interface IProps {
 }
 
 export default function LanguageSelector({ disabled }: IProps): JSX.Element {
-    if (SdkConfig.get()['disable_login_language_selector']) return <div />;
+    if (SdkConfig.get("disable_login_language_selector")) return <div />;
     return <LanguageDropdown
         className="mx_AuthBody_language"
         onOptionChange={onChange}