feat: make `j` rules immutable

[mrnagydavid]

In this PR, I showcase one pattern to make j immutable: with decorators on every method that mutates this.schema.

Why immutability? To avoid such issues:

export const mySchema = j.object({ ... })

// someone else
import { mySchema }
const schema = mySchema.optional() // <-- this makes `mySchema` optional **everywhere**