Add reactive thunk overload to preferences config option

[johanrd]

Summary

• Add thunk overload to preferences config option
• When preferences is a thunk, sync from adapter on each modify() cycle
• Enables reactive preference sharing between multiple tables

Usage

// Tables using thunks react to shared adapter changes                                                                                                                                                                                                                                                
headlessTable(this, {                                                                                                                                                                                                                                                                                 
  preferences: () => ({                                                                                                                                                                                                                                                                               
    key: "shared-table-preferences",                                                                                                                                                                                                                                                                                  
    adapter: this.args.sharedPreferencesAdapter,                                                                                                                                                                                                                                                            
  }),                                                                                                                                                                                                                                                                                                 
  // ...                                                                                                                                                                                                                                                                                              
});                                                                                                                                                                                                                                                                                                  

Test plan

• Added test for multi-table reactive sync
• All 186 existing tests pass