[TypeScript] Return type for euiSelectableTemplateSitewideFormatOptions

[thompsongl]

Summary

Closes #5616

Adds an explicit return type of EuiSelectableTemplateSitewideOption[] to euiSelectableTemplateSitewideFormatOptions to avoid a verbose inferred type resulting in hundreds of <div> and <li> DOM attributes and event handlers being added to the definition.

This change will eliminate ~600 lines from eui.d.ts

Checklist

• Checked for breaking changes and labeled appropriately
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5620/

[cee-chen]

I need to get better about return typing, if it can reduce inferred types/our eui.d.ts this much 🤯 Any thoughts on whether we should lint for it?

[thompsongl]

Any thoughts on whether we should lint for it?

I don't know about linting, but it would be nice to have file or bundle size stats for PRs so we get notified of large increases.

[cee-chen]

I just suggested linting because it seemed like an easy way/win to resolve the issue. It looks like we could enable this now: https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/explicit-function-return-type.md