Modernising CSS infrastructure in DevTools design doc

[Kriti Sapra]

Hi everyone, Please take a look at a proposal that outlines how we can modernise the CSS infrastructure in DevTools:

https://docs.google.com/document/d/1XtEFu6B-io7HpEZnlYZLHOUK8EtJMkoE3EjmEoJ5_1Y/ Best regards, Kriti

[John...@microsoft.com]

As a follow-up to this work, I have implemented a new ESLint rule to warn against non-import loading of CSS styles (calls to registerRequiredCSS) in-favor of using the new `import styles from '[file name].css.js;` (and *.css.legacy.js) syntax.

If you hit this warning the fix is to follow the example guidance and load the CSS styles with an import statement.
e.g. `import styles from 'mymodule.css.js';` and then later on `.registerCSSFiles([styles])`.

You may need to use the `*.css.legacy.js` suffix based on https://crrev.com/c/3275787

For more information see https://crrev.com/c/3277287 

Thanks to Kriti@ for starting this work and Jack@ for the reviews!