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

[trivikr]

Issue #, if available:
Fixes: #1919

Description of changes:
Use downlevel-dts to generate TS 3.4 compatible types.

Testing:

• Verified with tsc is successful while using typescript@3.7.5 in the repro code provided in Typescript 3.7.x breaking @aws-sdk/types #1919
• Verified that TS3.4 compatible types are created in lib/packages/clients:

clients/client-dynamodb

$ ls types                        
commands             endpoints.d.ts  pagination                  runtimeConfig.d.ts         ts3.4
DynamoDBClient.d.ts  index.d.ts      protocols                   runtimeConfig.native.d.ts  waiters
DynamoDB.d.ts        models          runtimeConfig.browser.d.ts  runtimeConfig.shared.d.ts
$ ls types/ts3.4                  
commands             endpoints.d.ts  pagination                  runtimeConfig.d.ts         waiters
DynamoDBClient.d.ts  index.d.ts      protocols                   runtimeConfig.native.d.ts
DynamoDB.d.ts        models          runtimeConfig.browser.d.ts  runtimeConfig.shared.d.ts

packages/types

$ ls types
abort.d.ts    credentials.d.ts  http.d.ts    middleware.d.ts  serde.d.ts      ts3.4
client.d.ts   crypto.d.ts       index.d.ts   pagination.d.ts  signature.d.ts  util.d.ts
command.d.ts  eventStream.d.ts  logger.d.ts  response.d.ts    transfer.d.ts
$ ls types/ts3.4
abort.d.ts    credentials.d.ts  http.d.ts    middleware.d.ts  serde.d.ts      util.d.ts
client.d.ts   crypto.d.ts       index.d.ts   pagination.d.ts  signature.d.ts
command.d.ts  eventStream.d.ts  logger.d.ts  response.d.ts    transfer.d.ts

lib/storage

$ ls types            
data-chunk  index.d.ts  ts3.4  upload  utils
$ ls types/ts3.4 
data-chunk  index.d.ts  upload  utils

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[aws-sdk-js-automation]

AWS CodeBuild CI Report

• CodeBuild project: sdk-staging-test
• Commit ID: 33d6ddb
• Result: SUCCEEDED
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[trivikr]

smithy-typescript PR smithy-lang/smithy-typescript#266

[AllanZhengYP]

Quote my comment on the Smithy PR for record:

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.

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