add .on() method to webln for events

[rolznz]

• webln:ready when webln is injected on the window
• webln:enabled when user enables webln

[bumi]

for what is the webln:enabled event?

either you call .enable()
or you can call isEnabled() - as per the other issue.

what do you need the webln:enabled for?

[rolznz]

@bumi

Example: Bitcoin connect does not know if webln.enable() is called from another part of the website than includes it. It probably shouldn't have to poll isEnabled.

[rolznz]

Update: webln provider should fire webln:enabled to the window/document. It shouldn't be a method on webln (window.webln might not even exist, then how can we subscribe?)