refactor: remove builtin import_ method from String class and inline module loading in compiler

The import_ method on String was a builtin that combined module loading and variable lookup. This change removes that method and instead emits separate bytecode instructions in the compiler: first calling loadModule_ on the module, then calling import_ on the module with the variable name. The native function string_lookUpVariable is renamed to string_import to match the new import_ method name on String's metaclass.

builtin/core.wren

@@ -58,12 +58,7 @@ class Sequence {
   }
 }
 
-class String is Sequence {
-  import_(variable) {
-    loadModule_
-    return lookUpVariable_(variable)
-  }
-}
+class String is Sequence {}
 
 class List is Sequence {
   addAll(other) {