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/utils/presence.ts

@@ -19,7 +19,7 @@ import SdkConfig from "../SdkConfig";
 
 export function isPresenceEnabled() {
     const hsUrl = MatrixClientPeg.get().baseUrl;
-    const urls = SdkConfig.get()['enable_presence_by_hs_url'];
+    const urls = SdkConfig.get("enable_presence_by_hs_url");
     if (!urls) return true;
     if (urls[hsUrl] || urls[hsUrl] === undefined) return true;
     return false;