Add autoExhaustive types

[zth]

Closes #590.

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Note

Adds project-wide exhaustive union/interface checks via autoExhaustiveTypes with opt-out @nonExhaustive, updates docs, and bumps rescript to 12.0.0.

• Compiler/Config:
  • Support autoExhaustiveTypes to auto-apply exhaustive checks for specified unions/interfaces.
  • Introduce @nonExhaustive directive to opt out (also disables auto checks for mutations when autoExhaustiveMutations is enabled).
• Docs:
  • Update docs/getting-started.md with autoExhaustiveTypes example in relay.config.js.
  • Expand docs/unions.md with project-wide exhaustive types and @nonExhaustive usage.
• Changelog:
  • Add entry noting autoExhaustiveTypes and @nonExhaustive support.
• Dependencies:
  • Bump rescript from 12.0.0-rc.3 to 12.0.0 and update corresponding yarn.lock platform/runtime packages.

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