Adds Browser.close()/1

[dbrody]

Allows for closing the whole browser, not just a page using Brower.close()/1

[coreyti]

@dbrody, thanks for contributing this. I've gone ahead with the merge, with the addition of a couple tweaks:

• I added a test of Browser.close/1, so we have at least one test explicitly covering the behavior.
• I updated the documentation for the function. It now more closely matches the docs from playwright.dev, with some small language tweaks.