Experiment: making TypeScript immutable-by-default

It's not expressly a TS thing, this will likely require a PR change to the linter/checker... maybe submit a request for a Frozen<T> type to be added to the core.

Aside, you can always use Object.freeze, but this can have odd side effects.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze