patch 8.2.4180: 'balloonexpr' is evaluated in the current script context

Problem:    'balloonexpr' is evaluated in the current script context.
Solution:   Use the script context where the option was set.

src/option.c

@@ -2614,6 +2614,21 @@ set_option_sctx_idx(int opt_idx, int opt_flags, sctx_T script_ctx)
     }
 }
 
+/*
+ * Get the script context of global option "name".
+ *
+ */
+    sctx_T *
+get_option_sctx(char *name)
+{
+    int idx = findoption((char_u *)name);
+
+    if (idx >= 0)
+	return &options[idx].script_ctx;
+    siemsg("no such option: %s", name);
+    return NULL;
+}
+
 /*
  * Set the script_ctx for a termcap option.
  * "name" must be the two character code, e.g. "RV".