Add `persist` and `retrieve` seed_types

[jkomoros]

A permanent version of let/var.

Should store in a simple key/value store in .profiles like AssociativeMemory.

Ensure it's inputValue (that is, only of types that are possible to persist in JSON)

The convention, like with environment, should be to prefix a domain you control to the front of the key: "komoroske.com:var"

• [x] Manually test store/retreive in filesystem mode
• [x] Delete should return true if the key existed, false otherwise
• [x] Return null if value doesn't exist