Combine `Expr::Wildcard` and `Expr::QualifiedWildcard`, add `wildcard()` expr fn

[alamb]

Is your feature request related to a problem or challenge?

As @waynexia points out on #8008 (comment), having Expr::Wildcard and Expr::QualifiedWildcard is confusing as they are both representing the same basic pattern, *

Specifically, I think it could make subtle bugs more likely (e.g. using Expr::Wildcard instead of using Expr::QualifiedWildcard)

Describe the solution you'd like

Combine Expr::Wildcard and Expr::QualifiedWildcard, so they can't be misused, and add wildcard() expr fn top

Describe alternatives you've considered

Do nothing

Additional context

No response