feat: add `doctest-annotation-spacing` ESLint rule

[Planeshifter]

Resolves https://github.com/stdlib-js/metr-issue-tracker/issues/127.

Description

What is the purpose of this pull request?

This pull request:

• add an ESLint rule to enforce spacing after a returns annotation (and also that there is only one space following //)

Related Issues

Does this pull request have any related issues?

No.

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• [x] Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• [x] Yes
• [ ] No

If you answered "yes" above, how did you use AI assistance?

• [x] Code generation (e.g., when writing an implementation or fixing a bug)
• [ ] Test/benchmark generation
• [ ] Documentation (including examples)
• [ ] Research and understanding

Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

PR was generated by Claude Code according to my specifications.

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