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/editor/serialize.ts

@@ -95,7 +95,7 @@ export function htmlSerializeIfNeeded(model: EditorModel, { forceHTML = false }
         patternNames.forEach(function(patternName) {
             patternTypes.forEach(function(patternType) {
                 // get the regex replace pattern from config or use the default
-                const pattern = (((SdkConfig.get()["latex_maths_delims"] ||
+                const pattern = (((SdkConfig.get("latex_maths_delims") ||
                     {})[patternType] || {})["pattern"] || {})[patternName] ||
                     patternDefaults[patternName][patternType];