Support URL in arbitrary values

[RobinMalfait]

This PR allows you to use bg-[url('...')] or stroke-[url(#...)]

It adds a new regex to detect the bg case, and it will also prevent adding spaces to operators like -.
This is important, otherwise this happens:

.bg-\[url\(\'\.\/path-to-file\.png\'\)\] {
  background-image: url('./path - to - file.png')
}

Fixes: #5197
Fixes: #5398