[REQ] New typescript-generator option to set file extension for ECMAScript module resolution

Created by: danmichaelo

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

I'm trying to generate a pure ESM package (type: "module") using ESM in Node.js, but the Typescript generators generate relative imports without file extensions (e.g. import * as runtime from "../runtime") while Node.js requires file extensions for ESM imports (import * as runtime from "../runtime.js")

Describe the solution you'd like

• The typescript generator already has an internal option extensionForDeno, which is set to .ts when platform is deno. I suggest renaming extensionForDeno to importFileExtension and making it a public option.
• The other typescript generators does not have such an option, so it may have to be added to those, if needed

I can try putting together a PR for this, but would like to gather some feedback about whether it's a good idea first.

Describe alternatives you've considered

• It's possible to generate a valid ESM package by setting moduleResolution: "node" (CommonJS for internal module resolution) and module: "ESNext", but this will fail if we add additional dependencies to the generated package that aren't CommonJS compatible.

• Instead of adding a new option importFileExtension, derive the file extension from some other option:

  • From platform? If we add a new value like node-esm. The problem is that there are several different extensions that can potentially be used (.js, .mjs, .tjs etc.) and users may have different preferences.
  • Perhaps a new option moduleResolution with values node and node16 inspired by TypeScript?