Fix issue of using the package as an ESM module

[mkermani144]

If we try to use this package as an ESM module in Node (without any build tool), we get errors like this:

SyntaxError: Named export 'Explorer' not found. The requested module '@ergolabs/ergo-sdk' is a CommonJS module, which may not support all module.exports as named exports.

This happens because the type or exports field is missing in package.json. Currently used module field in package.json is neither supported, nor the support is planned in Node. It was a proposal which was introduced by some bundlers like esbuild, but Node introduced a more powerful exports field.

Note that if this package is meant to be used only as an ESM module, adding a "type": "module" will suffice.