Add adaptive max_depth and max_items handling in diff_objects

[top-sigrid]

Fixes #35.

When comparing deeply nested structures or large arrays, diff_objects could return misleading "No changes. output because PrettyPlease truncates content beyond the passed max_depth & max_items limits. If both objects are truncated identically, no diff is shown even though the actual objects differ.

See also the issue in minitest-difftastic for details.

I concluded - based on our exchange in the minitest-difftastic issue, that this is best fixed here and minitest-difftastic actually doesn't need any adjustments.

This PR makes max_depth and max_items handling adaptive in diff_objects, increasing these, up until exceeding newly defined cap values, until either:

• A diff is found (strings differ after prettification) with higher max values
• Both caps are reached → returns "[Diff unavailable: exceeded depth/size display limits]"

Also more of these configurations are exposed in Difftastic::Differ.initialize for libraries and gems building upon this to customize this behaviour.