Updated package.json structs and populated new main field

[DoctorBracewell]

#1009 discusses how the generated package.json should have a main entry regardless of target type, this has been confirmed by a member of the NodeJS module team.

Added main field to NoModulesPackage and ESModulesPackage in the generated package.json.

Cloned the entry filename during struct instantiation for when the package.json is generated. Cloning might not be optimal, it is probably possible to borrow the String and generate the filename with that, but I think clones here should be fine.

• [✅ ] You have the latest version of rustfmt installed

$ rustup component add rustfmt

• [✅ ] You ran cargo fmt on the code base before submitting
• [✅ ] You reference which issue is being closed in the PR text

[zensh]

Look forward to merge, module field is not work in node.js.
And there should have "type": "module" in package.json. See https://github.com/yiwen-ai/isolang-wasm/blob/main/pkg/package.json