Add Pug syntax highlighting

[coyaSONG]

Summary

• add the Package Control Pug syntax as a pinned submodule
• add an MIT-attributed Pug syntax regression fixture and generated highlighting output
• update the existing Vue fixture to verify embedded Pug highlighting

Closes #1492.

Validation

• cargo test --locked
• rebuilt the official syntax assets with assets/create.sh, then rebuilt bat
• confirmed target/debug/bat --list-languages includes Pug:pug
• regenerated the full syntax-fixture set and confirmed the committed Pug and Vue outputs match

Disclosure

This change was implemented and validated by OpenAI Codex. It has not been reviewed by a human on the contributor side.