Add support for typescript isolatedDeclarations

[tompuric]

Description

Our monorepo utilises typescripts isolatedDeclarations in order to improve the efficiency of our projects type checking.

Currently the generated output from openapi-ts does not support isolatedDeclarations and produces errors such as

.../client/core/params.gen.ts(105,34): error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
.../client/client.gen.ts(16,14): error TS9010: Variable must have an explicit type annotation with --isolatedDeclarations.
.../client/sdk.gen.ts(3441,43): error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.

Plugins used:

• @hey-api/sdk
• @hey-api/typescript
• @hey-api/client-fetch

I believe openapi-ts should support this.

[pullfrog]

  ^{｜ Build this ➔ ｜ Make a plan ➔}

[tompuric]

I've provided a suggestive PR that works for us: #3880