fix(ButtonBase): memoize sx prop

[joshblack]

Closes https://github.com/github/primer/issues/1191

This PR tackles some of the small wins identified by @mattcosta7 over in https://github.com/github/primer/issues/1191

Specifically, it focuses on providing stable identity for StyledButton and the two underlying Box components in ButtonBase by:

• Pull out iconWrapStyles as a constant
• Pull out trailingIconStyles as a constant
• Create a default value for the sx prop, this allows it to be passed into sxStyles without triggering a new object each render

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: fa5a770d9ea7562fb631394b9f8ddab8d014947b

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[bot]]

size-limit report 📦

Path	Size
dist/browser.esm.js	77.21 KB (+0.03% 🔺)
dist/browser.umd.js	77.85 KB (+0.05% 🔺)