feat(astro-env): Allow setting descriptions on env schema config

[empoulter-uclan]

Changes

• Allows setting descriptions on env schema config
• Useful for describing where variables come from, how to generate them, etc.
• Description will be emitted in generated typescript definition file, for intellisense &c.

Testing

Additional config was added to a test fixture, and generated typescript definition was manually verified.

Docs

Probably could add a note in https://docs.astro.build/en/guides/environment-variables/#type-safe-environment-variables

[changeset-bot]

🦋 Changeset detected

Latest commit: 3c35be6

The changes in this PR will be included in the next version bump.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing empoulter-uclan:env-config-description (3c35be6) with main (3852adc)²}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (8a13969) during the generation of this report, so 3852adc was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩