Allow .mcfunction files to have defined permission levels

With the addition of the command confirmation screen, the need to allow non-operators to run some commands is necessary for map-makers and server owners. Therefore, I am proposing that .mcfunction files gain an optional header which defines the permission level necessary to execute them via /function. Here's an example .mcfunction file with a possible header:

#! permission 0
give @s diamond $(count)

In this case, this would allow normal players to execute a function to give themselves a specified amount of diamonds.

This would serve as a more modern and sophisticated alternative to /trigger and would allow custom dialogs to execute commands without needing a command execution warning, as the server or world owner would be the one to determine what functions are permitted to be run by users.

If the header is not present, it should assumed to be permission level 2, the current default.