Support duckdb_expression (new in 1.4.0)

New type: `duckdb_expression`
New functions:
- `duckdb_destroy_expression`
- `duckdb_expression_return_type`
- `duckdb_expression_is_foldable`
- `duckdb_expression_fold`

The only way to get a duckdb_expression is through these scalar function functions:
- `duckdb_scalar_function_bind_get_argument_count`
- `duckdb_scalar_function_bind_get_argument`