Better ESM integration with conditional exports

Since Conditional exports are not experimental anymore this will allow to use @metarhia/common as follows:

import common from '@metarhia/common'

Without the need for additional /module suffix.

[x] code is properly formatted (npm run fmt)
[x] tests are added/updated
[x] documentation is updated (npm run doc to regenerate documentation based on comments)
[x] description of changes is added under the Unreleased header in CHANGELOG.md

Just read through the docs and found out it's not experimental anymore and since we haven't released the current version this would IMO be better.

metarhia / common