Add type with show methods, text, svg and png support

[hellemo]

I started out fixing a minor error on julia 1.6, but figured the type and type display for QR codes from ShortCodes.jl would more naturally fit with the QRCodes package itself, hence this larger set of suggestions for changes:

• Rename to QRCodes to free QRCode for type name (and adhere to Julia package naming convensions)
• Add type QRCode for passing around and display in REPL/notebooks
  • store text and mode in type and add support for bitmap property (calculated when needed)
  • MIME image/svg+xml support (via Compose.jl)
  • MIME image/png support
  • MIME text/plain support (via UnicodePlots.jl)
• export QRCode as SVG or PNG file
• reduce number of exports