Specify which file not found in error (#11868)

# Description
Currently, `ShellError::FileNotFound` shows the span where the error
occurred but doesn't say which file wasn't found. This PR makes it so
the help includes that (like the `DirectoryNotFound` error).

# User-Facing Changes
No breaking changes, it's just that when a file can't be found, the help
will say which file couldn't be found:


![image](https://github.com/nushell/nushell/assets/45539777/e52f1e65-55c1-4cd2-8108-a4ccc334a66f)

crates/nu-protocol/src/shell_error.rs

@@ -727,8 +727,9 @@ pub enum ShellError {
     ///
     /// Does the file in the error message exist? Is it readable and accessible? Is the casing right?
     #[error("File not found")]
-    #[diagnostic(code(nu::shell::file_not_found))]
+    #[diagnostic(code(nu::shell::file_not_found), help("{file} does not exist"))]
     FileNotFound {
+        file: String,
         #[label("file not found")]
         span: Span,
     },