docs: create supplemental doc for typescript remarks (#5998)

supplemental-docs/TYPESCRIPT.md

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+# TypeScript
+
+The AWS SDK for JavaScript v3 is written in TypeScript. This allows easier generation of [API documentation](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/), and provides type hints and autocompletion to code editors, whether you are 
+using JavaScript or TypeScript.
+
+### Minimum supported TypeScript version
+
+The SDK does not have an official minimum supported TypeScript version. We recommend using a recent version of TypeScript. 
+The current version of TypeScript used in development of the AWS SDK can be seen in the root `package.json` file.
+
+## Common TypeScript-related questions in the AWS SDK for JavaScript (v3)
+
+#### Why are operation request and response structures unions with `| undefined`?
+
+This allows more flexibility when it comes to AWS service APIs changing over time without needing to update your SDK client version, and encourages defensive checking of API responses. 
+If you acknowledge the risks of not runtime-checking response object structures and would like to skip that, there is a `@smithy/types` type transform that can be applied to AWS SDK clients.
+
+For usage, see https://github.com/smithy-lang/smithy-typescript/tree/main/packages/types#scenario-removing--undefined-from-input-and-output-structures.
+
+#### Why are streaming output values a union type?
+
+The AWS SDK clients accept an optional runtime-provided value for `requestHandler` (see [requestHandler](https://github.com/aws/aws-sdk-js-v3/blob/main/supplemental-docs/CLIENTS.md#request-handler-requesthandler)).
+Because the implementation of the requestHandler varies both by platform (`node:https`-based for Node.js and `fetch`-based for browsers) and by service, and
+due to constraints from historical design decisions, the streaming response implementation is not precisely known at compile time.
+
+The type union describes why each member of the union exists ([source code](https://github.com/aws/aws-sdk-js-v3/blob/main/supplemental-docs/CLIENTS.md#request-handler-requesthandler) comments). 
+
+If you would like to narrow the type of streaming responses, for example for AWS S3 getObject, there is another type transform that can be applied to clients, with instructions at 
+`@smithy/types` https://github.com/smithy-lang/smithy-typescript/tree/main/packages/types#scenario-narrowing-a-smithy-typescript-generated-clients-output-payload-blob-types.