feat: add es2021 target

[luhc228]

Background

Currently, rax-compat export esm(ES Module + ES5) outputs by default, but it imports many helper functions from @swc/helpers and it will slow down the runtime like SSR. At this time, the node version installed in the server is 18.x or higher so it supports es2019 or higher syntax. For more detail, please see https://node.green/

What this PR do?

Now rax-compat will exports es2021 outputs. We can consume the es2021 outouts with bundlers.

For example, we can specify the resolve.conditionName in webpack config to import them:

export default {
  resolve: {
    conditionNames: ['es2021', '...']
  }
}

The base branch was changed.