Proposal: change how to detect metamask provider.

[uji]

The MetaMaskConnector detect provider from window.ethereum. (code)
This method has the problem that the provider can't be detected when use mobile.
Because, the connecter detect providers synchronously, but the MetaMask mobile provider is injected asynchronously.

Therefore, I would like to propose the use of @metamask/detect-provider.(recommended In docs.metamask.io)

It works out of the box in any modern browser, for synchronously and asynchronously injected providers.
ref: https://github.com/MetaMask/detect-provider#synchronous-and-asynchronous-injection

So I believe that we can also support the MetaMask mobile provider that is injected asynchronously and solve the problem.

[uji]

metamask/detect-provider README said.

The MetaMask extension provider is synchronously injected, while the MetaMask mobile provider is asynchronously injected.

But MetaMask/detect-provider#38 (comment) gave different answer.

Mobile is different because we don't provide MetaMask through a browser extension, we offer a separate app. Apps can't talk directly to each other, so if you have a dapp open in iOS Chrome, iOS Safari, Android Chrome, Android Firefox, etc., the site can't know that you have MetaMask installed, because no such script is injected on the page like it is in the browser on desktop.

It seems that using @metamask/detect-provider alone does not solve the mobile problem.