feat(cli): Improve the list command with options, and then some

1.
`rustlings list` should now display more than just the exercise names.
Information such as file paths and exercises statuses should be displayed.
The `--paths` option limits the displayed fields to only the path names; while the `--names`
option limits the displayed fields to only exercise names.
You can also control which exercises are displayed, by using the `--filter` option, or
the `--solved` or `--unsolved` flags.

Some use cases:
- Fetching pending exercise files with the keyword "conversion" to pass to my editor:
```sh
vim $(rustlings list --filter "conversion" --paths --unsolved)
```

- Fetching exercise names with keyword "conversion" to pass to `rustlings run`:
```sh
for exercise in $(rustlings list --filter "conversion" --names)
do
    rustlings run ${exercise}
done
```

2.
This should also fix #465, and will likely fix #585, as well.
That bug mentioned in those issues has to do with the way the `watch` command handler fetches the pending exercises.
Going forward, the least recently updated exercises along with all the other exercises in a pending state are fetched.

tests/fixture/state/info.toml

@@ -9,3 +9,10 @@ name = "pending_test_exercise"
 path = "pending_test_exercise.rs"
 mode = "test"
 hint = """"""
+
+[[exercises]]
+name = "finished_exercise"
+path = "finished_exercise.rs"
+mode = "compile"
+hint = """"""
+

tests/integration_tests.rs

@@ -181,3 +181,81 @@ fn run_single_test_success_without_output() {
         .code(0)
         .stdout(predicates::str::contains("THIS TEST TOO SHALL PAS").not());
 }
+
+#[test]
+fn run_rustlings_list() {
+    Command::cargo_bin("rustlings")
+        .unwrap()
+        .args(&["list"])
+        .current_dir("tests/fixture/success")
+        .assert()
+        .success();
+}
+
+#[test]
+fn run_rustlings_list_conflicting_display_options() {
+    Command::cargo_bin("rustlings")
+        .unwrap()
+        .args(&["list", "--names", "--paths"])
+        .current_dir("tests/fixture/success")
+        .assert()
+        .failure();
+}
+
+#[test]
+fn run_rustlings_list_conflicting_solve_options() {
+    Command::cargo_bin("rustlings")
+        .unwrap()
+        .args(&["list", "--solved", "--unsolved"])
+        .current_dir("tests/fixture/success")
+        .assert()
+        .failure();
+}
+
+#[test]
+fn run_rustlings_list_no_pending() {
+    Command::cargo_bin("rustlings")
+        .unwrap()
+        .args(&["list"])
+        .current_dir("tests/fixture/success")
+        .assert()
+        .success()
+        .stdout(predicates::str::contains("Pending").not());
+}
+
+#[test]
+fn run_rustlings_list_both_done_and_pending() {
+    Command::cargo_bin("rustlings")
+        .unwrap()
+        .args(&["list"])
+        .current_dir("tests/fixture/state")
+        .assert()
+        .success()
+        .stdout(
+            predicates::str::contains("Done")
+                .and(predicates::str::contains("Pending"))
+        );
+}
+
+#[test]
+fn run_rustlings_list_without_pending() {
+    Command::cargo_bin("rustlings")
+        .unwrap()
+        .args(&["list", "--solved"])
+        .current_dir("tests/fixture/state")
+        .assert()
+        .success()
+        .stdout(predicates::str::contains("Pending").not());
+}
+
+#[test]
+fn run_rustlings_list_without_done() {
+    Command::cargo_bin("rustlings")
+        .unwrap()
+        .args(&["list", "--unsolved"])
+        .current_dir("tests/fixture/state")
+        .assert()
+        .success()
+        .stdout(predicates::str::contains("Done").not());
+}
+