Issue with source maps

[mattfysh]

Hi kwhitley, thanks for this incredible library!

I noticed when installed from npm, the source map files (.map) reference non-existing source files, e.g. "sources":["../src/itty-durable.js"]

one other change I think can be made is replacing const { state, ...persistable } = this.getPersistable() with const persistable = this.getPersistable() as I think the get function already filters out the state property

cheers!