Require that FormRow has exactly one child element

[wylieconlon]

Summary

EuiFormRow requires exactly one child element to work, which was too loose at runtime and compile time. This adds stricter type checks and a proptype check. The error messages are now much easier to follow:

Closes #1931

Screenshots showing updated errors:

Checklist

• [ ] This was checked in mobile
• [ ] This was checked in IE11
• [ ] This was checked in dark mode
• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
• [ ] This was checked against keyboard-only and screenreader scenarios
• This required updates to Framer X components

[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?

[thompsongl]

jenkins test this

[thompsongl]

jenkins test this

[thompsongl]

LGTM. Don't forget to add a changelog entry

[wylieconlon]

retest

[ryankeairns]

Thanks @wylieconlon !!