feat: allow string width input

[aqilaziz]

Summary

• widen the width input type to number | string so strict Angular templates accept values such as width="100%"
• keep the QRCode renderer config numeric by passing undefined for non-numeric widths
• update README parameter docs for the new accepted type

Testing

• npm run lint -- --fix=false
• npm run ci:build:lib
• git diff --check

Fixes #324