Rename to "Module Declarations" and update readme

[nicolo-ribaudo]

This is parallel to https://github.com/tc39/proposal-module-expressions/pull/77: the two proposals are now named "Module Expressions" and "Module Declarations", to better explain their relationship and their differences.

[Jack-Works]

I'm wondering if we can merge those two proposals into one. Module Expression + Module Declaration + Import Module Declaration syntax => Reified module

[littledan]

@Jack-Works I prefer to leave them separate for now, as module expressions is a much simpler proposal and works on its own and so would come first. They are designed to compose well; I am glad that we have this design fully sketched out.