ex_getln: Refactor script_get()

1. Use `char *` for strings.
2. Add `const` qualifiers.
3. Add attributes and documentation.
4. Handle skipping *inside*.
5. Handle non-heredoc argument also inside: deferring this to the caller is
   pointless because all callers need the same thing. Though new ex_lua caller
   may live without allocations in this case, allocating nevertheless produces
   cleaner code.
6. Note that all callers call script_get with `eap` and `eap->arg`. Thus second
   argument is useless in practice: it is one and the same always and can be
   reached through the first argument.

src/nvim/ex_docmd.c

@@ -3734,10 +3734,12 @@ void ex_ni(exarg_T *eap)
 /// Skips over ":perl <<EOF" constructs.
 static void ex_script_ni(exarg_T *eap)
 {
-  if (!eap->skip)
+  if (!eap->skip) {
     ex_ni(eap);
-  else
-    xfree(script_get(eap, eap->arg));
+  } else {
+    size_t len;
+    xfree(script_get(eap, &len));
+  }
 }
 
 /*