Assign permissions by game function

[zml2008]

Currently the /function command runs functions with an elevated op level of at least 2 -- we should allow this to be a permissions-configurable thing.

I'm not sure if this wants to be implemented as a context, or as a new subject type -- leaning towards context since executed in a function modifies the original sender, and is generally temporary.

Also, how do we deal with tags? Given a specific function, can we see which tags contain the function?

Functions: https://minecraft.gamepedia.com/Function_(Java_Edition)