Add credo NoOpenBrowser check

[ftes]

resolves #95

This should work - using on a different project. Haven't thought about how to unit test this.

[ftes]

I've added tests. Notes:

• bad: also reports on other open_browser calls (import Phoenix.LiveViewTest, defp open_browser(conn))

So it gets the job done for most cases, but could be improved.

[germsvel]

@ftes thanks for all the work on this. I'm going to rebase this branch and get it merged.