Add optional importing of variables/procedures

[phobos2077]

Need some ways to check if variable was exported before fetching/storing (and possibly crashing the script). This should allow for global scripts to communicate without hard requirements.

Concrete motivation for this: my damage mod script needs to read/write some data for other global script to use. But if these external vars doesn't exist, it can work normally as a stand-alone damage mod, because it doesn't depend on them.

[phobos2077]

So there's 2 ways to approach this (at least):

1. Add new opcode/metarule to check if given exported var exists:

import variable super_var;
//...
value := super_var
   if external_var_exists(@super_var) // can use stringify operator here to not confuse ssl optimizer
   else 0;

2. Change vanilla op_fetch_external to just print error and return 0 but continue script. op_store_external similarly can just print error and continue. Not sure how "correct" this is but it will solve the main issue. You can check against 0 to see if some variable was set or not.

@dekrus thoughts?