API: nvim_source: save/restore script context #11159

Use a constant for the script id.

src/nvim/globals.h

@@ -331,6 +331,7 @@ EXTERN int garbage_collect_at_exit INIT(= false);
 #define SID_NONE        -6      // don't set scriptID
 #define SID_LUA         -7      // for Lua scripts/chunks
 #define SID_API_CLIENT  -8      // for API clients
+#define SID_STR         -9      // for sourcing a string
 
 // Script CTX being sourced or was sourced to define the current function.
 EXTERN sctx_T current_sctx INIT(= { 0 COMMA 0 COMMA 0 });