Provide function for getting webln instance without enabling

[wbobeirne]

For folks who want to detect if a user has a webln client, and treat them differently without triggering an enable call, they currently have to check for window.webln. Rather than have those checks littered throughout apps and making it different to alternative methods of webln injection, we should provide a function that does that for them. Something like getProvider. For the sake of future-proofing for those alternative cases, it may be best to keep the function asynchronous, though that may be frustrating for some use-cases.