feat: add support for stdin and stdin-filename options

[SirSam21]

Closes #803

✔ Checklist:

• [ ] Commit messages have been written in Conventional Commits format;
• [x] Tests for the changes have been added (for bug fixes / features);
• [ ] Docs have been added / updated on the dotenv-linter.github.io (for bug fixes / features).

[SirSam21]

I wanted to add that some of this code may look awkward because I tried to make stdin fit into the project structure that focuses on traversing directories and reading files directly. I'm hoping to get some input from others on how this might be addressed.

[SirSam21]

I removed --stdin-filename and fixed the tests I made. For the tests I pretty much just had to add the with_default_args() to avoid the update check during testing.

[codecov-commenter]

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Codecov Report

:x: Patch coverage is 97.05882% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 99.06%. Comparing base (5b6de3a) to head (9f03ec3). :warning: Report is 43 commits behind head on master.

Files with missing lines	Patch %	Lines
dotenv-linter/src/lib.rs	95.45%	2 Missing :warning:
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #829      +/-   ##
==========================================
- Coverage   99.15%   99.06%   -0.09%     
==========================================
  Files          46       48       +2     
  Lines        2834     3205     +371     
==========================================
+ Hits         2810     3175     +365     
- Misses         24       30       +6     

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