EuiHighlight: converted to Typescript

[ffknob]

Summary

Closes #2666
Converted EuiHighlight to Typescript.

Checklist

• Check against all themes for compatability in both light and dark modes
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[chandlerprall]

Couple changes requested, also needs a CHANGELOG.md entry

[chandlerprall]

Remove these propTypes, they will be generated automatically from the TS definitions.

[ffknob]

So the goal is to move all propTypes to the component's interface? (just to be sure before I take the other convertions)

[ffknob]

• CHANGELOG: added (thought code refactory was not to be put there, my mistake)
• HTMLSpanElement: corrected
• children: included in the component's Props as string

One doubt about propTypes as general (so I can take on the other convertions): should I completly get rid of it and move all attributes to the Props interface?

[chandlerprall]

One doubt about propTypes as general (so I can take on the other convertions): should I completly get rid of it and move all attributes to the Props interface?

Yes, the React propTypes duplicate the information provided by the typescript definition, so we don't want to include them in the components. However, it is useful to provide the proptypes when components are used in non-TS projects, so we generate them (based on the TS definitions) at build time. Our docs also use the resulting proptypes to populate any props tabs.

[chandlerprall]

One final nit pick, otherwise this looks good

[chandlerprall]

className is already provided by CommonProps and doesn't need to be re-specified, common.ts#L10

[chandlerprall]

Changes LGTM, pulled and tested locally

[chandlerprall]

Thank you once again, @ffknob !