Add npm repository and bugs metadata

[wowsofine]

Summary

• add standard npm repository metadata pointing to this GitHub repo
• add standard npm bugs.url metadata pointing to the repo issues page
• leave runtime code, version, dependencies, and build config unchanged

Verification

• npm view markdown-it-amsmath@0.4.0 name version homepage repository bugs deprecated dist.tarball scripts --json
• metadata smoke for repository.url and bugs.url
• npm pack --dry-run --ignore-scripts
• npm ci --ignore-scripts
• npm run build
• npm test
• npm run lint
• npm pack --dry-run --ignore-scripts
• git diff --check