* 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
