fix: Fix ESM distribution for node import usage

[isaacs]

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:

import reactElementToJSXString from 'react-element-to-jsx-string'

TypeScript users would have to do this:

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.

[armandabric]

@isaacs I will have look. It looks like the smoke test suite it not really well done for modern ESM build