feat: publish both ESM and CommonJS

[pvditto]

Currently, the library is distributed as a commonjs module, which at this point is starting to cause issues with frameworks that only support ES modules. To solve this, both a commonjs and esm version is exported.

Build output and package.json changes

This PR updates the published package.json to use an exports field to define conditional exports dependent on the method of importing. The main field, and a new module field, in package.json are kept as a fallback. The commonjs and esm sources are published in separate directories, using separate Typescript configs, and using new scripts configured in package.json. Running the npm run build script still runs all required steps to create a distributable package.

As ESM packages need to explicitly set a type field in their package.json marking them as such, a helper script injects a minimal additonal package.json into the two cjs and esm subdirectories. Otherwise, importing the esm version would fail as it would be considered a commonjs module by default in many tools.

Typescript Configuration Changes

The Typescript-exported sourcemaps are updated to inline the original module source so that source map and original source don't have to be included both in order to facilitate source mapping.

Supported ES syntax is updated to support the latest syntax for writing react-ditto by using the EXNext configuration.

debatable: The target ES version of the build output is updated from ES5 to ES6. Unless we have it at ES5 for a particular reason, I feel that ES6 support is wide spread enough at this point and some ES6 features (WeakMap) are required for Ditto to work anyway.

Caveats

In ES modules, import statements need to include an unambiguous reference to a source. As the published package uses .js files, the file ending of the TS sources needs to match that, even though the .js files don't even exist yet. Unfortunately, tsc can't transform this during build (background).

[jreyes33]

LGTM, thanks! I totally agree with building for ES6.

[pvditto]

@jreyes33 thanks for the review. I am going to merge this. Let's be extra careful with the next release as this is changing a couple of essential pieces of project configuration and build output. Let me know when it's time and I can test the published library a bit.