fix(Button): make children prop optional

[joshblack]

Closes https://github.com/github/primer/issues/2601

Update our base ButtonProps so that children is optional. This allows for the following migration pattern to be supported for v36:

// Before
<Button leadingIcon={ChatIcon}>
  <Button.Counter>{5}</Button.Counter>
</Button>

// After
<Button leadingAction={ChatIcon} count={5} />

[changeset-bot]

🦋 Changeset detected

Latest commit: 6f0270a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	100.69 KB (0%)
dist/browser.umd.js	101.27 KB (0%)

[broccolinisoup]

🚀