Extension permanent data storage

[Doomsdayrs]

Create an interface so that extensions can permanently store simple data in Shosetsu.

I will discuss with @TechnoJo4 regarding this

[TechnoJo4]

A general version of this would be pretty hard to get right. Should consider usecases and then figure out which specific features are necessary.
For temporary storage, people can already just create local variables on the top-level of their files.