Use built-in functions in a custom Engine, without instantiating RecalcEngine

We want to create our own custom Engine instead of using RecalcEngine, but we'd still like to use some of the built-in functions provided in the internal library:

https://github.com/microsoft/Power-Fx/blob/2a82bc542d94d9ffd9e5bc81ce46c30dca53e8dd/src/libraries/Microsoft.PowerFx.Interpreter/RecalcEngine.cs#L51

Can we provide something like ReadOnlySymbolTable.NewDefault() to allow us to obtain the functions without creating RecalcEngine as a throwaway, intermediate resource?

Or even better, perhaps something like SymbolTable.NewDefault(), so we can modify the set without having to call GetMutableCopyOfFunctions first to selectively remove the ones we don't want.

If there's a problem with it because SymbolTable is in Core and the library is in Interpreter... then maybe a CreateNewDefaultSymbolTable() in PowerFxConfigExtensions.cs

Thanks :)