feat: add Button component

[jaredcwhite]

Issue Overview

This PR addresses #36

Description

This PR adds a Button component which covers the following use cases:

• Render an HTML <button> which defaults to type button, or can be set to type submit (or reset) for form submissions
• Render an HTML <a href="…"> which links to an internal page (but still visually looks like a button)
• Render an HTML <a href="…" target="_blank"> which links to an external page and shows a box+arrow icon

In addition, there's a "borderless" variant which lets a button appear visually as a link.

How Can This Be Tested/Reviewed?

Storybook link: https://deploy-preview-42--storybook-ui-seeds.netlify.app/?path=/story/actions-button--default-button

Checklist:

• [ ] I have added QA notes to the issue with applicable URLs
• [x] I have performed a self-review of my own code
• [x] I have reviewed the changes in a desktop view
• [x] I have reviewed the changes in a mobile view
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have exported any new components
• [x] My commit message(s) and PR title are polished in the conventional commit format, and any breaking changes are indicated in the message and are well-described

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[jaredcwhite]

@emilyjablonski Other than the question of the link icon being optional or default, I've addressed your recommendations.

[jaredcwhite]

Note: I'd forgotten to prefix the component-level tokens with button- to stay in line with existing Seeds conventions, so I've updated the PR with that.

[github-actions[bot]]

:tada: This PR is included in version 1.8.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[slowbot]

LGTM!