feat: add use cases package

[travis]

Add an example of how to delegate all capabilities on a space to a did:mailto account. This has come up a few times recently and it's one potential solution to the "I want to change my email address" problem.

I'd like to start collecting use cases for w3up client here. It might be a good idea to run some or all of them as integration tests.

[alanshaw]

I made this https://github.com/storacha-network/w3up-examples (very similar timing lol).

I don't really care much for whether they are part of this repo or separate, but they should all be together.

Note: I'd probably lean more towards separate repo in order to not have loads of frontend dependencies or other dependencies used only in examples when npm installing.