Feat: correctly handle the generation of VALUES expressions using macros

[georgesittas]

This PR aims to address a feature gap. This is the behavior in main today:

>>> from sqlglot import parse_one
>>> from sqlmesh.core.macros import MacroEvaluator
>>>
>>> MacroEvaluator().transform(parse_one("SELECT * FROM (VALUES @EACH([1, 2, 3], v -> (v)) ) as v")).sql()
'SELECT * FROM (VALUES ((1), (2), (3))) AS v'
>>> MacroEvaluator().transform(parse_one("SELECT * FROM (VALUES (@EACH([1, 2, 3], v -> (v))) ) as v")).sql()
'SELECT * FROM (VALUES ((1), (2), (3))) AS v'

Notice how both inputs result in the same output, which produces a single row with three columns. There is no obvious way to produce three columns, using @EACH, due to how the VALUES parsing works today.

[themisvaltinos]

lgtm. I imagine this preserves any other functionality and in this first position it should be each always this macro function?

[georgesittas]

It doesn't have to be @EACH– other macro functions could also return lists. In that case, we should interpret the items in the list as either columns or rows depending on the presence of the parentheses after VALUES.