Feature: lamdera check --only-preserve-backend

[tibastral]

Hello, I would love to have a way to do a lamdera check --only-preserve-backend
As I don't care very much about other migrations for now (since I don't have many users), it would make my work very easier.

Thank you so much !

[CharlonTank]

What I do is the following: run lamdera check and only fill the backendModel function. Other functions can be either ModelUnchanged or MsgUnchanged.