Export `TimestampToEventResponse` to use in `matrix-react-sdk` tests (#3223)
* Export ITimestampToEventResponse to use in matrix-react-sdk tests
Part of https://github.com/matrix-org/matrix-react-sdk/pull/10405
* Remove I from interface
As suggested by @weeman1337,
https://github.com/matrix-org/matrix-js-sdk/pull/3223#pullrequestreview-1350612347
See code style guide, https://github.com/vector-im/element-web/blob/50f8be4a623d2280a72920eb1679a4754961f807/code_style.md#typescript--javascript-typescript-javascript
> Interface names should not be marked with an uppercase `I`.