Implement EXCEPT and REPLACE in select item

[apstndb]

This PR implements EXCEPT and REPLACE in SelectItem.

Note:

• There is no breaking changes.
• Introduced p.lookaheadStarModifierExcept() to avoid ambiguity between * EXCEPT (columns) and * EXCEPT ALL or * EXCEPT DISTINCT.
• The prefix StarModifier is borrowed from ZetaSQL: https://github.com/google/zetasql/blob/194cd32b5d766d60e3ca442651d792c7fe54ea74/zetasql/parser/gen_parse_tree.py#L3617-L3634

fixes #184

[makenowjust]

IMO, we can reuse Alias here instead of introducing a new struct.

[apstndb]

Thank you for your comment. I agree that we can reuse Alias for it.

I've been avoiding using ast.Alias for a bit since ast.Alias is used for both of AS optional and AS required and it is not natural to implement parseAlias().
May be it is better to introduce parseAlias(requiredAs withAs).

• May we better to consider to rename Alias to AliasedExpr? Introduce TypelessStructLiteralArg and NewConstructorArg interfaces #183 (comment)

[apstndb]

IMO, it is better to separate between expr AS alias and expr [AS] alias.
Currently they are not compatible with parseCommaSeparatedList unless we have a no-arg named parse* function.

Additionally, It seems StarModifierReplace is the first structure having expr AS name[, ...].
IMO, this is reason enough to have a separate StarModifierReplaceItem at this point.

[makenowjust]

Thank you!

[makenowjust]

Good catch.