[typescript] Add importFileExtension option to support ECMAScript module resolution

[danmichaelo]

Node.js ECMAScript module resolution requires file extensions for relative imports. Typescript also conforms to this requirement when using the new nodenext module resolution setting.

File extension support was already added to the Typescript(experimental) generator for Deno in #6732. The current PR generalizes the extensionForDeno parameter into importFileExtension and makes it user configurable, so users can set it to .js for a pure ESM package or .mjs for a dual package.

The current PR only adds the option to TypeScriptClientCodegen, which seems to be the new generator if I understand it right, but if the PR is accepted, I'd like to try adding it to AbstractTypeScriptClientCodegen as well in a separate PR, to support typescript-fetch and friends.

fixes #14333

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[macjohnny]

Thanks for your contribution

[julientechdev]

Thank you for this PR, adding support for typescript-fetch would be nice 👍

[florianbepunkt]

Support for the typescript axios generator would be nice as well.

[coeing]

@danmichaelo Would it be possible to move the importFileExtension property to AbstractTypeScriptClientCodegen? This would make it available in other typescript code generators, like typescript-fetch and typescript-axios as well.