Add "fall-through" signatures (#7527)

Fixes https://github.com/nushell/nushell/issues/4659
Fixes https://github.com/nushell/nushell/issues/5294
Fixes https://github.com/nushell/nushell/issues/6124
fix https://github.com/nushell/nushell/issues/5103

crates/nu-protocol/src/ast/call.rs

@@ -7,6 +7,7 @@ use crate::{DeclId, Span, Spanned};
 pub enum Argument {
     Positional(Expression),
     Named((Spanned<String>, Option<Spanned<String>>, Option<Expression>)),
+    Unknown(Expression), // unknown argument used in "fall-through" signatures
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
@@ -36,6 +37,7 @@ impl Call {
         self.arguments.iter().filter_map(|arg| match arg {
             Argument::Named(named) => Some(named),
             Argument::Positional(_) => None,
+            Argument::Unknown(_) => None,
         })
     }
 
@@ -46,6 +48,7 @@ impl Call {
         self.arguments.iter_mut().filter_map(|arg| match arg {
             Argument::Named(named) => Some(named),
             Argument::Positional(_) => None,
+            Argument::Unknown(_) => None,
         })
     }
 
@@ -64,10 +67,15 @@ impl Call {
         self.arguments.push(Argument::Positional(positional));
     }
 
+    pub fn add_unknown(&mut self, unknown: Expression) {
+        self.arguments.push(Argument::Unknown(unknown));
+    }
+
     pub fn positional_iter(&self) -> impl Iterator<Item = &Expression> {
         self.arguments.iter().filter_map(|arg| match arg {
             Argument::Named(_) => None,
             Argument::Positional(positional) => Some(positional),
+            Argument::Unknown(unknown) => Some(unknown),
         })
     }
 
@@ -75,6 +83,7 @@ impl Call {
         self.arguments.iter_mut().filter_map(|arg| match arg {
             Argument::Named(_) => None,
             Argument::Positional(positional) => Some(positional),
+            Argument::Unknown(unknown) => Some(unknown),
         })
     }