Added signString() method

[robwoodgate]

Adds optional signString() method specified in nostr-protocol/nips#1842
Fixes KeyPair.ts filename case that was causing build errors

[robwoodgate]

Will close #13

[neilck]

I like the fact that its signing a hash that it calculates. This is much safer than previous general signing proposals where people could pass in anything. I might put in some checking to ensure the message isn't a nostr event trying to bypass permission checks. I'll merge it in in the next couple of days.

[robwoodgate]

Thanks - yes, I thought it was important the user has a chance to see what they are signing, rather than a random hash.

Also returning all the elements needed to verify what was signed and by whom.

I have used the same implementation I gave to nos2x, but the raw message string input also gives you scope to expand the method later to check the message string for a pubkey and sign using that user rather than selected user, as you have done for the signEvent()

[robwoodgate]

Hey @neilck - I've added a string and validateEvent security check. If you want me to rebase into a single commit, let me know.