Create stacks caip19.md

[friedger]

This PR

• adds a definition for CAIP 19 for the Stacks blockchain

[bumblefudge]

Hey @friedger , glad to see you back! Sorry for slow turnaround time, I'm on family leave, but will still try to get this merged in a reasonable time frame.

[bumblefudge]

@friedger i'm back! Did you have a chance to review my suggestions?

[bumblefudge]

this is incredibly embarassing to admit (immutably and publicly, no less) but my three in-line comments above were sitting in draft unsent for... a year, thanks to a github.com UX change that passed me by unnoticed. sincere apologies if you got notifications that made no sense referring to unseen comments.