Upgrade packages tests exports

[keithamus]

This upgrades this package to fall in line with the improvements made in our most recently updated Web Component, the relative-time-element.

The development changes are:

• Uses web-test-runner over karma.
• Uses a slightly improved eslint config
• Minor changes to TSconfig
• Uses esbuild over rollup

User faces changes are:

• Emits JSX types, making it compatible with React
• Reworks exports allowing for various patterns, including importing the web component without defining, or defining under different scopes or registries.
• Outputs a custom elements manifest.

Refs https://github.com/github/primer/issues/2138

[primer-css]

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