Implement `--check` for `visudo`

[pvdrz]

Describe the changes done on this pull request This PR adds the required logic to implement the --check/-c flag for visudo.

Blocked by #689.

Pull Request Checklist

• [x] I have read and accepted the code of conduct for this project.
• [x] I have tested, formatted and ran clippy over my changes.
• [x] I have commented and documented my changes.
• [x] This pull request will partially issue https://github.com/memorysafety/sudo-rs/issues/657 where a proper discussion about a solution has taken place.

[github-actions[bot]]

Number of dependencies and binary size impact report

Metric	main	PR #690	Delta
Direct dependencies	3	3	-
Total dependencies	4	4	-
Binary size	996.6 KiB	996.4 KiB	-
Text size	580.3 KiB	580.5 KiB	-

Dependencies diff

```diff └─ sudo-rs [v0.2.0-dev.20230711] ├─ glob [v0.3.1] ├─ libc [v0.2.147] └─ log [v0.4.19] ```

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.26 :warning:

Comparison is base (fad6b90) 57.53% compared to head (b70b946) 57.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #690 +/- ## ========================================== - Coverage 57.53% 57.27% -0.26% ========================================== Files 68 68 Lines 9035 9076 +41 ========================================== Hits 5198 5198 - Misses 3837 3878 +41 ``` | [Impacted Files](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/690?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety) | Coverage Δ | | |---|---|---| | [src/visudo/mod.rs](https://app.codecov.io/gh/memorysafety/sudo-rs/pull/690?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=memorysafety#diff-c3JjL3Zpc3Vkby9tb2QucnM=) | `0.00% <0.00%> (ø)` | |

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