ESM Support

[goastler]

Hi, due to changes in module resolution using ESM with typescript, the .js extension is now required for relative imports and assert { type: "json" } for json imports.

Could you add a parameter of some description to enable these extensions/assertions for the output of typechain? Would be mighty handy in our case where our codebase is in ESM typescript.

We're currently doing a regex substitution to add the extensions and assertions, but it's super hacky!