fix: add dist package.json to files

[antstanley]

Node.js uses either file extensions or the "type" field in package.json to determine if files are CommonJS or ES Module files. When we shipped dual module support, package.json files were placed in both dist/esm and dist/cjs with the correct "type" property. This would enable consumers of the library to correctly interpret what type of module system the files were using.

The problem is that we did not add these dist specific sub package.json files to the files property of the root package.json so they were never published. This led to Node assuming all files were ESM, even the ones in dist/cjs.

This leads to errors for folks trying to use CommonJs. This PR adds those files into the files property to resolve this issue.

Resolves #615

[naorpeled]

Thanks @antstanley !!!