Improve validator bundling

[Siegrift]

https://api3dao.atlassian.net/browse/AN-622

Added build targets for non cjs targets. This makes it more optimal to load the validator inside the browser environment (although modern bundles would probably handle cjs as well).

I've tested this using a simple FE application and validator published locally (using verdaccio). I've also made sure the source maps works.

[amarthadan]

Just to make sure, we'll be still able to use

import { parseConfigWithSecrets } from '@api3/airnode-validator';

and not something like

import { parseConfigWithSecrets } from '@api3/airnode-validator/dist/cjs/...';

[Siegrift]

Yes. We will for all things that are exported in via the main index.js file