fix(api): don't override Vimscript SID (#32610)

Problem:  When calling an API from Vimscript to set an option, mapping,
          etc., :verbose shows that it's set from an API client.
Solution: Don't override current_sctx.sc_sid when calling an API from
          Vimscript. Also fix the inverse case where API channel id is
          not set when calling an API from RPC. Move channel id into
          sctx_T to make saving and restoring easier.

Related #8329

src/nvim/option_defs.h

@@ -191,5 +191,5 @@ typedef struct {
   opt_expand_cb_T opt_expand_cb;
 
   OptVal def_val;                    ///< default value
-  LastSet last_set;                  ///< script in which the option was last set
+  sctx_T script_ctx;                 ///< script in which the option was last set
 } vimoption_T;