When loading this module with import() in Node, the CommonJS module was
being loaded, resulting in {default:{default:fn}} instead of just
{default:fn} as intended.
Because the types are only defined for CommonJS, this was further
compounded, meaning that instead of doing this:
```ts
import reactElementToJSXString from 'react-element-to-jsx-string'
```
TypeScript users would have to do this:
```ts
const {
 default: { default: reactElementToJSXString },
} = (await import('react-element-to-jsx-string')) as unknown as {
 default: typeof import('react-element-to-jsx-string')
}
// wat.
```
- Add conditional exports for import and require, so that the ESM is
 served to import() users.
- Add a build script to place a package.json file in dist/esm, setting
 it to ESM mode for Node's loader and TypeScript's module resolution.
- Test that require() and import() receive matching objects, when
 loading from a mjs file in a folder outside this project.
With this change, `import()` in Node loads the ESM version, properly
typed, without any hassle.