Better Thunderbolt cable labels

Improvements

• Cable speed labels now mention Thunderbolt class. USB4 Gen 3 cables show "Thunderbolt 4 class" and Gen 4 cables show "Thunderbolt 5 class" so you can tell at a glance whether your cable supports TB5 signalling. Thanks to @applefarmer for the report in #131.

• Passive e-marker on Thunderbolt cables gets a clarifying note. Some TB4 cables (CalDigit, Cable Matters) report as passive in USB-PD because their active components condition the Thunderbolt signal path, not the USB path. When the Thunderbolt link is live, WhatCable now explains that Thunderbolt is negotiated separately by the controller. Thanks to @pandoratactful for the report in #111.