Add environment variable expansion to @source directive

[hookenz]

I'm using the standalone build of tailwindcss in a golang project with templ. Specifically templui.
I pull in the components using go's package manager. The components are outside the project in a path

${GOPATH}/pkg/mod/github.com/axzilla/templui

So I have to manually expand the GOPATH variable here:

@source "${GOPATH}/pkg/mod/github.com/axzilla/templui@*/**/*.{go,templ}";

It would be especially nice if this could be automatic. It would help support other languages like golang.

An alternative and is to support templ and golang directly. That would be absolutely amazing but probably quite complex and getting a bit too specific. I think the variable expansion is the simplest approach for now.