Convert to ES Modules

[JuanM04]

Closes #66

arg now exports a CJS and ESM version. For compatibility reasons, I've avoided using any export default, resulting in this breaking change:

const arg = require('arg')

// Before
const args = arg(/* ... */)

// After
const args = arg.run(/* ... */)

Note: that change can be avoided if the package goes ESM-only, but that seem too radical

@leerob let me now what you think about these changes

[JuanM04]

I've added a pnpm build step in CI

[jorenbroekema]

Could this perhaps be looked at? I'm happy to help. @leerob