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),
         })
     }
 

crates/nu-protocol/src/signature.rs

@@ -118,6 +118,7 @@ pub struct Signature {
     pub allow_variants_without_examples: bool,
     pub is_filter: bool,
     pub creates_scope: bool,
+    pub allows_unknown_args: bool,
     // Signature category used to classify commands stored in the list of declarations
     pub category: Category,
 }
@@ -220,6 +221,7 @@ impl Signature {
             is_filter: false,
             creates_scope: false,
             category: Category::Default,
+            allows_unknown_args: false,
         }
     }
 
@@ -274,6 +276,12 @@ impl Signature {
         self
     }
 
+    /// Allow unknown signature parameters
+    pub fn allows_unknown_args(mut self) -> Signature {
+        self.allows_unknown_args = true;
+        self
+    }
+
     /// Add a required positional argument to the signature
     pub fn required(
         mut self,