Make error-message more helpful when user invokes a non-executable file (#13589)

# Description

Fixes Issue #13477 
This adds a check to see if a user is trying to invoke a
(non-executable) file as a command and returns a helpful error if so.
EDIT: this will not work on Windows, and is arguably not relevant there,
because of the different semantics of executables. I think the
equivalent on Windows would be if a user tries to invoke `./foo`, we
should look for `foo.exe` or `foo.bat` in the directory and recommend
that if it exists.

# User-Facing Changes

When a user invokes an unrecognized command that is the path to an
existing file, the error used to say:
`{name} is neither a Nushell built-in or a known external command` 
This PR proposes to change the message to:
`{name} refers to a file that is not executable. Did you forget to to
set execute permissions?`

# Tests + Formatting
Ran cargo fmt, clippy and test on the workspace. 
EDIT: added test asserting the new behavior

tests/shell/pipeline/commands/external.rs

@@ -127,6 +127,15 @@ fn command_not_found_error_suggests_typo_fix() {
     assert!(actual.err.contains("timeit"));
 }
 
+#[cfg(not(windows))]
+#[test]
+fn command_not_found_error_recognizes_non_executable_file() {
+    let actual = nu!("./Cargo.toml");
+    assert!(actual.err.contains(
+        "refers to a file that is not executable. Did you forget to to set execute permissions?"
+    ));
+}
+
 #[test]
 fn command_not_found_error_shows_not_found_1() {
     let actual = nu!(r#"