v0.1.0 of invoice_kit

Generate customizable invoice, bill, and receipt PDFs in Flutter with multiple templates, JSON-driven layouts, GST support, QR codes, and cross-platform export. Changelog excerpt: Initial production release of invoice_kit. ### Features - 🎨 5 built-in templates: Classic (B2B with GST), Modern (minimalist), Compact (events/retail), Thermal (80mm POS), JSON-driven (fully dynamic) - 📋 Dual data API: Strongly-typed Invoice models OR raw JSON InvoiceData for server-driven flows - 💰 GST-ready with CGST/SGST/IGST breakdown, multi-currency (default INR), multi-locale number formatting - 🔤 Full Unicode support (₹, Hindi, Tamil, Arabic, Chinese, etc.) via Google Fonts (Roboto,[...]