[EuiTab] Fix standalone tabs not defaulting to a size

[cee-chen]

Summary

When we converted EuiTabs to Emotion in #6311, we started passing down size to child EuiTab, but we didn't set a default size = 'm' for <EuiTab>s used outside of parent <EuiTabs> or when consumers add, e.g. a wrapper of some kind around EuiTab.

This is affecting a Kibana plugin and will need to be backported:

⚠️ Note: Technically this fix does not address scenarios where consumers do something like this:

<EuiTabs size="l">
  <SomeIntermediateWrapper>
    <EuiTab> // will be size "m" / will not inherit size "l"

For the above scenarios, we need to use React Context instead to share size, similar to how EuiDescriptionList behaves, instead of cloning children with props. However, as that's a significantly more involved fix and Kibana needs a working fix sooner rather than later, I'm opting to prefer this fix for the backport and look into a more future-proof context fix later.

QA

General checklist

• Added or updated jest snapshots
• A changelog entry exists and is marked appropriately

[breehall]

This looks good to me! Context will make this a little easier down the road. Great work Constance!

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6366/