Refactor `GM.*Value(s)` functions and storage structure

[ACTCD]

Based on the work of #331, we can refactor the GM.*Value(s) functions in a similar way:

• Data for the one script will be stored in only one key (maybe)
• Extensions should provide an interface for users to inspect the use of persistent storage by user scripts and provide a way to clean them up https://github.com/quoid/userscripts/issues/319.
• Provides read and write methods similar to native browser.storage (possibly, via a new proxy wrapper)