fix path exists on a non-directory file (#13763)

# Description
Fixes:  #13460

The issue is caused by `try_exists` method on path, it will return
`Err(NotADirectory)` if user tried to check for a path under a regular
file.
To fix it, I think it's ok to use `exists` rather than `try_exists`,
although
[Path::exists()](https://doc.rust-lang.org/std/path/struct.Path.html#method.exists)
only checks whether or not a path was both found and readable. I think
it's ok, and we can add this information under `extra_description`.

# User-Facing Changes
The following code will no longer raise error:
```
touch a; 'a/b' | path exists
```

# Tests + Formatting
Added 1 test.

crates/nu-command/tests/commands/path/exists.rs

@@ -64,6 +64,18 @@ fn const_path_exists() {
     assert_eq!(actual.out, "true");
 }
 
+#[test]
+fn path_exists_under_a_non_directory() {
+    Playground::setup("path_exists_6", |dirs, _| {
+        let actual = nu!(
+                cwd: dirs.test(),
+                "touch test_file; 'test_file/aaa' | path exists"
+        );
+        assert_eq!(actual.out, "false");
+        assert!(actual.err.is_empty());
+    })
+}
+
 #[test]
 fn test_check_symlink_exists() {
     use nu_test_support::{nu, playground::Playground};