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.
diff --git a/src/proto/option.pro b/src/proto/option.pro
index a099624..11f342c 100644
--- a/src/proto/option.pro
+++ b/src/proto/option.pro
@@ -22,6 +22,7 @@
 void redraw_titles(void);
 int valid_name(char_u *val, char *allowed);
 void set_option_sctx_idx(int opt_idx, int opt_flags, sctx_T script_ctx);
+sctx_T *get_option_sctx(char *name);
 void set_term_option_sctx_idx(char *name, int opt_idx);
 void check_redraw(long_u flags);
 int findoption(char_u *arg);