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/widgets/Jitsi.ts

@@ -70,7 +70,7 @@ export class Jitsi {
 
     private update = async (discoveryResponse: IClientWellKnown): Promise<any> => {
         // Start with a default of the config's domain
-        let domain = SdkConfig.get().jitsi?.preferredDomain || "meet.element.io";
+        let domain = SdkConfig.getObject("jitsi")?.get("preferred_domain") || "meet.element.io";
 
         logger.log("Attempting to get Jitsi conference information from homeserver");
         const wkPreferredDomain = discoveryResponse?.[JITSI_WK_PROPERTY]?.['preferredDomain'];