Create a guide to help devs migrate from v3 to v4

[cedoor]

Tasks

• List code differences
• Migration strategy

Links

• https://eslint.org/docs/latest/use/migrate-to-8.0.0

[artwyman]

FYI Zupass is going to be tackling this migration soon, so I'll stay in touch about our strategy.
The most interesting part is probably our desire to derive a V4 identity from a V3 identity. The likely default there is to do something (append, hash, xor) with the V3 secrets to derive a V4 secret, but we have some people thinking about whether there are things we can do which are more clever (but still secure) which might allow us to ZK prove that a given V4 identity was derived from a V3 identity.