add global scope variables

[CajunAvenger]

• Adds the -> operator, variable_name -> variable_definition
• Defining or redefining a variable with the -> operator causes it to become "globally scoped" rather than dynamically scoped. Changes to the variable at any level are retained, letting games/styles cache more expensive operations or use more permanent storage.
• Once a variable is globally scoped, it is always. variable := value will be valid and update it globally.
• I tried to make it so if a variable was initially declared with := instead of ->, it would be unable to be global, but i was unable to make it work

[haganbmj]

Merged in the rest of Gen's fork so this got wrapped up in that.
#96

[CajunAvenger]

This wasn't in Gen's fork yet:
we didn't want to release it on 1 day of testing, so we only added the -> token, which just acts as :=
that way if a user uses mse 2.5.6 to run the code meant for the next version, it will just be feature incomplete rather than potentially crash