Add tab index option

[craigmulligan]

Description of changes

I needed a way to add some predictability when tabbing through features on a complex map, are you interested in supporting this functionality?

It works by using the feature.properties.tabindex property & passing that through to the button.

For author

• [ ] Cite changes under the HEAD tag in CHANGELOG.md
• [x] Add tests for new functionality

For reviewer

• [ ] Run npm test to confirn browser tests pass
• [ ] Confirm tests cover all new functionality