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/@types/common.ts

@@ -41,3 +41,11 @@ export type RecursivePartial<T> = {
         T[P] extends object ? RecursivePartial<T[P]> :
             T[P];
 };
+
+// Inspired by https://stackoverflow.com/a/60206860
+export type KeysWithObjectShape<Input> = {
+    [P in keyof Input]: Input[P] extends object
+        // Arrays are counted as objects - exclude them
+        ? (Input[P] extends Array<unknown> ? never : P)
+        : never;
+}[keyof Input];

src/@types/global.d.ts

@@ -52,7 +52,7 @@ import { ConsoleLogger, IndexedDBLogStore } from "../rageshake/rageshake";
 import ActiveWidgetStore from "../stores/ActiveWidgetStore";
 import { Skinner } from "../Skinner";
 import AutoRageshakeStore from "../stores/AutoRageshakeStore";
-import { ConfigOptions } from "../SdkConfig";
+import { IConfigOptions } from "../IConfigOptions";
 
 /* eslint-disable @typescript-eslint/naming-convention */
 
@@ -63,7 +63,7 @@ declare global {
         Olm: {
             init: () => Promise<void>;
         };
-        mxReactSdkConfig: ConfigOptions;
+        mxReactSdkConfig: IConfigOptions;
 
         // Needed for Safari, unknown to TypeScript
         webkitAudioContext: typeof AudioContext;