Upgrade Astro to v4 and refactor tests to ESM

[delucis]

• Upgrades Astro and related dependencies to v4
• Refactors tests to .mjs files so we don’t need tsm to run them

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: cef976b9f21180cde0f0204a6e5535c3706dd909

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