feat: create no-duplicate-definitions

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Hello,

I’ve finished implementing no-duplicate-definitions.

I’ve left some comments on some notable parts of the code.

What changes did you make? (Give an overview)

I’ve finished implementing no-duplicate-definitions.

Related Issues

closes: #358

Is there anything you'd like reviewers to focus on?

[lumirlumir]

// is usally used as a definition-style comment.

References:

• https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md053---link-and-image-reference-definitions-should-be-needed
• https://stackoverflow.com/questions/4823468/comments-in-markdown
• https://www.jamestharpe.com/markdown-comments/

[nzakas]

LGTM. Thanks! Would like @fasttime to review before merging.

[lumirlumir]

@nzakas @fasttime @snitin315

Thanks for all your reviews! I've pushed a few more commits.

[nzakas]

LGTM, thanks! Would like @fasttime and @snitin315 to verify their feedback before merging.

[snitin315]

LGTM, thanks! Just a small suggestion, totally optional and not blocking the merge.

[nzakas]

Re-approving.