Add trace()

[MGatner]

Description
We advertise trace() in the User Guide as a debug tool wrapping Kint::trace() but it isn't actually there. This PR adds it to Common.php.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[lonnieezell]

Userguide actually shows, Kint::trace(); as how to use it, but I'm fine adding this feature in. Just need to update the docs.

[MGatner]

Ah I see - the subheaders misled me since the other two are global functions. Well it’s fine either way but if this seems good I’ll update that doc.

[lonnieezell]

yeah, I think it's worth doing.

[MGatner]

@lonnieezell All set!