Include an ESM version of ts-morph in the npm package

**Is your feature request related to a problem? Please describe.**

ts-morph is currently transpiled to CommonJS, so it can't be treeshaked by webpack (https://webpack.js.org/guides/tree-shaking/).

**Describe the solution you'd like**

It'd be great if the ts-morph package also included an esm bundle, so it can be used in esm modules and treeshaked!

**Describe alternatives you've considered**

Start by reading CONTRIBUTING.md and reviewing how the npm package is currently produced. Use the webpack tree-shaking guidance linked in the issue to understand the ESM requirement. Done means the package includes an ESM bundle that works in ESM modules and can be tree-shaken.

Written by the indexing model from the issue text.