This has annoyed me for some time, needing to remember the filter
pushdown logic is defined in `arrow_array_stream.cpp` which is
completely illogical

Cleaned it up, added a Makefile for `make format-main`, and extended the
import cache to get rid of some raw `py::_module::import` calls

This PR fixes
https://github.com/duckdb/duckdb/issues/19018#issuecomment-3299362636

description in tests

Fixes duckdb#74 

This PR:
- exports the `adbc_driver_duckdb` package so you can use it after `pip
install duckdb`
- doesn't just skip the tests if the package can't be found
- limits the exported symbols to the only two that are ever needed:
`duckdb_adbc_init` and `PyInit__duckdb`:

❯ nm -g -C -P
.venv/lib/python3.11/site-packages/_duckdb.cpython-311-darwin.so | ag -v
' U '
        _PyInit__duckdb T 41900 0
        _duckdb_adbc_init T 1054e6c 0

Fixes duckdb#57 

- Created and curated new stubs
- Added py.typed
- Added mypy config
- Copied the typing and functional modules into sqltypes and func and deprecated the originals, to prevent shadowing issues

See duckdb#98 

We have our own Polars IO plugin to create lazy polars dataframes. We
try to push as many predicates down into DuckDB as we can, for which we
try to map Polars expressions (including datatypes) to SQL expressions.
To do this we depend on Polar's `Expr.meta.serialize`.

None of this is very stable. Polars IO source plugins are marked
`@unstable` and `Expr.meta.serialize` says "Serialization is not stable
across Polars versions".

In this case the problems seems to come from Polars requiring an
explicit scale to be set for decimals ([this
pr](pola-rs/polars#24542)). The serialized
format seems to have changed into:
```json
{
  "expr": {
    "Literal": {
      "Scalar": {
        "Decimal": [
          1,
          38, // This now includes the scale
          0
        ]
      }
    }
  },
  "dtype": {
    "Literal": {
      "Decimal": [
        38, // this was already there
        0
      ]
    }
  },
  "options": "Strict"
}

```

Interestingly, even if we explicitly set precision to e.g. 20, the
relevant part of the serialized expression looks as follows:
```json
{
  "expr": {
    "Literal": {
      "Scalar": {
        "Decimal": [
          1,
          38, // Still 38?
          0
        ]
      }
    }
  },
  "dtype": {
    "Literal": {
      "Decimal": [
        20, // This is correct
        0
      ]
    }
  },
  "options": "Strict"
}
```

This PR allows for both a 2 and 3 item list for decimals.