Add errors for unknown experimental keys

[TheOtterlord]

Changes

Adds validation for experimental keys, throwing a custom zod error if any unknown keys are present.

[astro] Unable to load /home/otterlord/Documents/Repos/astro/examples/basics/astro.config.mjs

 error   Invalid experimental key: `notARealExperiment`. 
  Make sure the spelling is correct, and that your Astro version supports this experiment.
  See https://docs.astro.build/en/reference/configuration-reference/#experimental-flags for more information.

Preview

Testing

Manually tested

Docs

The error links to existing related documentation. (see related docs)

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Latest commit: 289f398

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[bluwy]

Forgot to followup on this. Seems like a nice addition!