Define explicit return types to prevent invalid relative paths in d.ts files during TS compilation

[YusukeHirao]

Fixes #1548

What is the purpose of this PR?

• Fix bug
• Fix typo
• Update specs
• Add new rule
• Add new parser
• Improve or refactor rules
• Add a new core feature
• Improve or refactor core features
• Update documents
• Others

Description

Checklist

Fill out the checks for the applicable purpose.

• Fix bug
  • Add the test that can check whether resolved the issue.
• Update specs
  • Element
    • Update global attributes
    • Update content models
    • Add also ARIA that you explored ARIA in HTML Recommendation and Editor's draft
  • Attribute
    • Update global attributes
    • Update IDL attribute
• Add new rule
  • Add a test
  • Add a README document
    • Create an issue that requests translating the doc if you want.
  • Add to index
  • Add JSON schema to schema.json
  • Add to @markuplint/config-presets
    • And add to Rulesets of base presets
      • Add to a doc file
      • Add to a doc file (ja), or create an issue that requests translating the doc if you want.
  • Add to the Playground
    • Add to a config object
• Add new parser
  • Add a test
    • Do a test with some parser.
  • Add to initializer