[tsconfig.json] Consider setting "importHelpers:false"

The root `tsconfig.json` currently sets `importHelpers: true`:

https://github.com/Azure/azure-rest-api-specs/blob/f6bf6555a71cb3167dcff04cc7964bda9ae36a88/tsconfig.json#L19

This is probably unnecessary, and it requires that TypeScript projects in this repo include a dependency on `tslib` (ideally a direct dependency, but may be currently satisfied by a transitive dependency).

We should try changing this to `importHelpers: false`. I believe the root `tsconfig.json` is currently only used by this script:

https://github.com/Azure/azure-rest-api-specs/blob/f6bf6555a71cb3167dcff04cc7964bda9ae36a88/scripts/prettier-swagger-plugin.ts

As documented here:

https://github.com/Azure/azure-rest-api-specs/blob/f6bf6555a71cb3167dcff04cc7964bda9ae36a88/documentation/ci-fix.md#prettier-check

Related: #24756