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-command/tests/commands/exec.rs

@@ -0,0 +1,58 @@
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn basic_exec() {
+    Playground::setup("test_exec_1", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                nu -c 'exec nu --testbin cococo a b c'
+            "#
+        ));
+
+        assert_eq!(actual.out, "a b c");
+    })
+}
+
+#[test]
+fn exec_complex_args() {
+    Playground::setup("test_exec_2", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                nu -c 'exec nu --testbin cococo b --bar=2 -sab --arwr - -DTEEE=aasd-290 -90 --'
+            "#
+        ));
+
+        assert_eq!(actual.out, "b --bar=2 -sab --arwr - -DTEEE=aasd-290 -90 --");
+    })
+}
+
+#[test]
+fn exec_fail_batched_short_args() {
+    Playground::setup("test_exec_3", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                nu -c 'exec nu --testbin cococo -ab 10'
+            "#
+        ));
+
+        assert_eq!(actual.out, "");
+    })
+}
+
+#[test]
+fn exec_misc_values() {
+    Playground::setup("test_exec_4", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                nu -c 'let x = "abc"; exec nu --testbin cococo $x [ a b c ]'
+            "#
+        ));
+
+        assert_eq!(actual.out, "abc a b c");
+    })
+}