feat: use downlevel-dts to generate TS 3.4 compatible types

[trivikr]

Issue #, if available:
Fixes: aws/aws-sdk-js-v3#1919

Description of changes:
use downlevel-dts to generate TS 3.4 compatible types for clients in AWS SDK for JavaScript

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[trivikr]

aws-sdk-js-v3 aws/aws-sdk-js-v3#1943

[AllanZhengYP]

Should this be in the AWS repo only? I feel like other SDK implementer should not use this.

[trivikr]

Should this be in the AWS repo only? I feel like other SDK implementer should not use this.

If we put this code in smithy-typescript, it would a recommendation from AWS to support old versions of TypeScript type definitions. And I would like to have that explicit recommendation.

Without this recommendation, the SDK implementers will not be aware that their TypeScript types may not work with old versions of TypeScript. The implementer is always free to remove postbuild script, downlevel-dts dependency and typesVersions key.

[AllanZhengYP]

OK I was assuming the downleveled *.d.ts will be shipped to all users, that will compromise the user experience, until to understand typesVersion. According to the TypeScript doc. The consumers will only use the downleveled *.d.ts when their local TypeScript version is older. The consumers using newer TypeScript can still use the newer features. It's just the built client will generate and be shipped with multiple versions of *.d.ts.

It has some overhead regarding the download package size. But they don't goes to the .js file and it helpers users able to support broader development environments, outweights the drawbacks.

[AllanZhengYP]

Approve again