feat(formatters): add severity custom attribute to TeamCity inspections

[Vampire]

Checklist

• [x] Tests added / updated
• n/a Docs added / updated

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Additional context

This PR adds the SEVERITY custom attribute to the inspection TeamCity service messages. It is true that this is just a custom attribute, but it the one shown as example in the TeamCity documentation, and it also is just half-custom, because SEVERITY="ERROR" is indeed considered by TeamCity when segregating the reported inspectinos into warnings and errors.