feat(miniflare): add getWorkerdFeatureFlags() for compatibility flag resolution

[petebacondarwin]

Add a TypeScript implementation that replicates workerd's C++ compileCompatibilityFlags() function. This computes which compatibility flags are enabled based on:

• Compatibility date (flags enabled after their compatEnableDate)
• Explicit compatibility flags (enable/disable overrides)
• Implied flags (cascading dependencies between flags)

Files added:

• scripts/build-capnp-compat.mjs: Build script that downloads compatibility-date.capnp from workerd, compiles it with capnp-es, and extracts annotation metadata
• src/shared/compatibility-date.ts: Cap'n Proto types generated by capnp-es
• src/shared/compatibility-flags-metadata.ts: Extracted annotation metadata
• src/shared/compatibility-flags.ts: Main implementation with getWorkerdFeatureFlags()
• test/shared/compatibility-flags.spec.ts: Test suite

Also updates:

• package.json: Added capnp:compat script
• tsconfig.json: Include the build script for type checking
• GitHub workflow: Auto-regenerate compatibility flags on workerd dependency updates

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because:

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Latest commit: 0d12508

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commit: 0d12508

[vicb]

YYYY-MM-DD is a comparable string, do we need to parse?