S6.2.7 extended keyword checks

A while back we added extended keyword rules https://klc.kicad.org/symbol/s6/s6.2/

This MR adds extended keyword checks:

• Check for duplicate keywords `opamp single opamp?
• Check for filler words in keywords
• Check for duplicate keywords in sub-tokens analog devices analog-devices
• Check for keyword (sub-)tokens which already appear in description (sub-)tokens

Unit tests are added as well: