feat(watch): show hint while watching
`rustlings hint ...` command is not convenient when doing exercises with `rustlings watch`.
This PR makes it possible for user to type `hint` while running `watch` and get hint text for exercise which is currently failing.
e.g.
```rust
...
--> exercises/variables/variables1.rs:13:36
|
13 | println!("x has the value {}", x);
| ^ not found in this scope
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0425`.
type 'hint' to get help:
hint
Hint: The declaration on line 12 is missing a keyword that is needed in Rust
to create a new variable binding.
```