[REQ] [typescript-fetch] Support ECMAScript modules

Is your feature request related to a problem? Please describe.

I generate code, which I want to use in a project where I use ECMAScript modules instead of CommonJS (https://nodejs.org/api/esm.html#enabling). A problem here is, that a project using ECMAScript modules requires every import to point to the exact location of the file to import, in which case the path must include the extension of the file name (https://nodejs.org/api/esm.html#mandatory-file-extensions).

This lead me to a stackoverflow question (https://stackoverflow.com/questions/62619058/appending-js-extension-on-relative-import-statements-during-typescript-compilat) where the way to solve it was described as adding the extension to the path.

Describe the solution you'd like

Adjust all import-paths to point to the exact location, so a project, having set moduleResolution to node16 in the compiler options of the tsconfig.json file can compile.

Describe alternatives you've considered

Currently I have to switch back to using CommonJS for my library instead of ECMAScript modules, which would be the preferred option.

Additional context

The ECMAScript section is not all too new (stable since node v14), but I'd still consider this a feature and not a bug, as it is an additional option of generating valid typescript code.

Start at the typescript-fetch generator entry point and trace how it produces import paths. Use the tsconfig.json setting with moduleResolution set to node16 as the validation context, and verify that generated imports include the exact file extensions required for ECMAScript modules.