Adds a line to the composer pull-down that will insert:
[wrap=qrcode url=]
[/wrap]
and will render a QR Code. If you leave url= blank, it will use the current page URL. You can also type whatever text you want in after url= to have that text rendered in the QR Code (tip: It doesn't have to be a URL, but does need to be enclosed in "quotes").
It seems that due to limitations of the library you can have only one QR Code per post.
Uses code from https://davidshimjs.github.io/qrcodejs/.
Thanks to @falco and the discourse-mermaid-theme-component for inspiration and Best Practices.
See https://github.com/literatecomputing/discourse-qrcode-theme-component which has the QR Code text in a code block rather than a [wrap]. That version is more rubust and recommended, but this version serves as an example for how to manipulate a [wrap] block in a theme component.