Replace `injectFile` to `injectScript` and add `injectCSS`

When browsing trough the docs and using the API, I noticed that Puppeteer has an injectFile method. The name might be confusing, because you might think it works for a CSS file as well. I think it would be better to rename this method to injectScript and also add a injectCSS method.

Just an idea though 😃.

[aslushnikov]

@vsemozhetbyt injectFile injects from file system, whereas addStyleTag adds a url. I’d rather unify the two methods since it’s confusing.

Something like:

page.addScriptTag({
  url: '...', // either URL or PATH could be specified
  path: '..',
});

[SamVerschueren]

addScriptTag and addStyleTag currently already work with a local file path, that’s what the tests do as well. Not sure if it’s a good idea though, that’s probably the reason why injectFile exists.

If we would go with your solution, an object with url and path, it might be nice to have a content option as well.

page.addScriptTag({
  url: '...',
  path: '..',
  content: `document.write('Hello World')`
});