[standards] Reserve op_return prefix for eCash Chat platform

Summary: Reserves the prefix 63686174 (chat) for messaging transactions generated from the eCash Chat platform. This is to enable other apps like Cashtab to parse and render them accordingly.

Test Plan: plonk prefix guideline into https://jbt.github.io/markdown-editor/

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Subscribers: tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D15672