feat: custom file handlers

[noomorph]

This is a relatively huge refactor, which contains substantial fixes to the API design in many places:

1. Adding runtime plugins API, specifically for handling certain custom attachments.
2. Unified interface for accessing any step metadata:

• obscure differences between squashed (in reporter) and actually stored (at runtime) have been removed;
• users can visit nested steps and distinguish between custom steps and Jest's hooks;
• docblocks can be used publicly, from the reporter, and their processing can be customized.

[github-actions]

🎉 This PR is included in version 2.0.0-beta.5 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

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