feature: support of global changeable variables

[Roba1993]

Hi all,

I'm switching from rhai to rune because of the async support of rune. One feature I'm really missing are global changeable variables.

In my case it's needed because I have up to 30 different inputs for the engine which all can change within the engine (by rhai code from the user) and need to be read out at the end again.

My actual workaround is to have a struct which is the input of each function and need to be the output. But this is not really user friendly and prevents additional use-cases where I want to return function specific data in addition.