js-yaml: more strict loader return types

[FredKSchott]

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes:
  • https://github.com/nodeca/js-yaml#safeload-string---options-
  • https://github.com/nodeca/js-yaml/blob/573ba47ba1a70b56ae0e60e04ed23164f0c05341/lib/js-yaml/loader.js#L1573
• Increase the version number in the header if appropriate.
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Rationale:

A typescript consumer should be responsible for typing the output of load(), safeLoad(), etc. However, with the current any return type, it's easy for the user to assume that these methods always return an object or an array of objects. With this fix, TS will now warn if the user defines their own return type without handling for undefined.

[typescript-bot]

@FredKSchott Thank you for submitting this PR!

🔔 @Bartvds @sclausen - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

After 5 days, no one has reviewed the PR 😞. A maintainer will be reviewing the PR in the next few days and will either merge it or request revisions. Thank you for your patience!