Add createScalarFunction into AsyncDuckDB/AsyncDuckDBConnection

[kolmakova]

DuckDB-WASM has a function called createScalarFunction in DuckDBConnection to create used-defined functions. However, in the current version (1.28.1-dev179.0) I haven't been able to find a way to use it because AsyncDuckDB and AsyncDuckDBConnection don't implement this method. It seems to have been just forgotten to implement. Or are there some reasons why we can't use it in AsyncDuckDB and AsyncDuckDBConnection?

[rossbailey-trace]

This would be great to have!