Add a stacktrace-inducing template token

[ribasushi]

Having err objects respond to %+v is quite widespread within the golang ecosystem. Add a logger template unit supporting this behavior.

[zubairalam]

Hi @aldas . Can I pick this if it needs to cleaned?

[ribasushi]

@aldas apologies for the radio silence. Was not tested originally as it wasn't clear what would substitute a reasonable test. Please observe this playground for the real-world-use motivation behind this change.

Would you accept a test using stacked xerrors.Errorf(... %w) ?