[feature] diffs in failures

[ngarbezza]

Is your feature request related to a problem? Please describe.
When an assertion fails, we got a message explaining the problem. But sometimes, this is not enough or it is hard to spot where the problem is, because the message is one line mostly with toString() applied to the involved objects, and printed in a single line.

Describe the solution you'd like
A new setting, called "failureDiffs", that when enabled, displays after the test failure and the message, a new section called "diff", with the actual and expected values colored where the differences are, pretty much like git diffs, but can be less sophisticated.

Additional context
We prefer simplicity over introducing a new dependency. As it needs to remain cross platform, do not rely on OS diff commands. Any important decision made during this feature has to be tracked using an Architecture Decision Record.