Convert Guardian from 'old' CommonJS (CJS) modules to 'new' ECMAScript (ESM) modules

[anvabr]

Problem description

As of Node.js v12+ it natively supports ESM which are also supported by all modern browsers. Guardian interfaces multiple external systems, uses 3rd party libraries. We have already encountered a CJS/ESM incompatibility issue with w3up web3.storage libraries, and anticipate that such problems will occur increasingly more often.

Requirements

• Convert Guardian existing CJS build system into ESM.
• Note in the documentation that Guardian requires ESM modules (for those wishing to extend or modify Guardian codebase).

Definition of done

Guardian codebase is built with ESM technology.

Acceptance criteria

No impact on fictionality or clients.