Positional Reference Operator

[Mytherin]

This PR introduces the positional reference operator (#) that can be used to refer to specific columns of the FROM clause by position, rather than by name.

For example:

CREATE TABLE integers(i integer, j integer);
INSERT INTO integers VALUES (1, 2), (2, 2), (3, 2);
SELECT #1 FROM integers;
┌───┐
│ i │
├───┤
│ 1 │
│ 2 │
│ 3 │
└───┘

While this can be used from SQL, its primary use-case is for use in the relational API during ad-hoc analysis where column names are generally less accessible, e.g.:

import duckdb
duckdb.values([1, 2]).project('#1 + #2').df()
#   #1 + #2
# 0        3