Expose Matrix.User

[BramvdnHeuvel]

This pull request adds the Matrix.User module to be used by the developer.

Additionally, it adds a breaking change where the sender field of the public Event type is now a User type rather than a sender, effectively pushing for a major change.

[BramvdnHeuvel]

I have looked through the code and it looks great to me. The documentation also renders nicely.

One idea for the future, however, is to be a bit more strict on combining the strings in the Text module.

[BramvdnHeuvel]

Similarly, the ParserExtra.elm might need a refactor in the future into something similar to the current Json module, where it is rebuilt to serve the purpose of the Elm SDK better. Right now, the parsers offer little explanation about what is wrong with the strings parsed.

Given how there's issues like matrix-org/matrix-spec#1506 floating around the Matrix ecosystem, it might benefit debugging if the module was more helpful on how certain strings like user IDs do not meet the requirements of the Matrix spec.