- created an export for P_SpawnMissile so that I can do some tests with functions.

- allow class extensions.

These are separate blocks in different files that get concatenated to one class body for processing. The reason is to allow spreading the many functions in Actor over multiple files, so that they remain manageable. For example, all the Doom action functions should be in their respective files, but their symbols need to be in Actor. To extend a class, both files need to be in the same translation unit, so it won't allow user-side extension of internal classes.

src/scripting/zscript/zcc_parser.h

@@ -32,6 +32,7 @@ enum
 	ZCC_ReadOnly		= 1 << 9,
 	ZCC_FuncConst		= 1 << 10,
 	ZCC_Abstract		= 1 << 11,
+	ZCC_Extension		= 1 << 12,
 };
 
 // Function parameter modifiers