feat(python)!: Improve consistency of parsing expression input

[stinodego]

!! BREAKING !!

These are some improvements aimed at better handling inputs for select/with_columns and other methods that take expression inputs. These have already been deprecated - this is the PR that actually changes the behaviour.

Changes:

• Passing None as the first input is now parsed as lit(None) rather than be ignored.
• Passing an iterable (e.g. select(["a", "b"])) is still supported, but can no longer be combined with other positional parameters. If multiple positional parameters are supplied, the first argument is always assumed to be an expression input, rather than a list of expression inputs.
• Methods that accept named expression inputs (select, with_columns, agg, struct) no longer have a separate argument for the first expression input. This means that select(exprs="a") now renames the column "a" to "exprs".