custom and JS converters

[jraymakers]

• Add custom result data conversion methods to both DuckDBResult and DuckDBResultReader:
  • convertColumns
  • convertColumnsObject
  • convertRows
  • convertRowObjects

Expose types and utility functions for working with custom converters.

• Add methods for converting result data to built-in JS types:
  • getColumnsJS
  • getColumnsObjectJS
  • getRowsJS
  • getRowObjectsJS

Re-implement JSON converter to use new infrastructure (using a map of functions).

Add tests for the new JS conversions.