BREAKING CHANGE: This PR removes the TypeScript declaration file down-leveling feature from the JSII compiler to address significant performance issues. Our benchmarks show that >50% of the JSII compile time is spent in the downlevel-dts dependency, causing JSII to be 2.7x slower than standard TypeScript compilation. The down-leveling feature was originally added to support TypeScript versions down to 3.9, but analysis shows minimal usage - none of our AWS CDK codebase uses this feature, and only about 12 non-fork repositories on GitHub would be affected by its removal, additionally typescript 3.9 and earlier have been EOL for quite a while on definitively typed.

If you found this pr because removing this feature impacted your builds, it is likely because you have this pattern somewhere in your codebase. Simply refactor that pattern and rebuild. Ensure you also are not seeing the below typesVersion in your app's package.json

  "typesVersions": {
    "<=3.9": {
      "*": [
        ".types-compat/ts3.9/*",
        ".types-compat/ts3.9/*/index.d.ts"
      ]
    }
  },

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