Specify `"type": "module"` when deploying to nodejs-module

[kawaemon]

WIP because this PR depends on #4091

When --target experimental-nodejs-module is specified, Either using .mjs extension or setting "type": "module" field on package.json is required to execute emitted javascript code using Node.js.

In this PR, I used "type": "module" approach because it makes lower impact on existing codebases since it doesn't change file name.