Simpler CSS import for TS views and CSS @import support

[Haprog]

This modifies the default webpack loader configuration for all *.css files. Instead of the previous raw-loader only, this will use a loader chain of ['lit-css-loader', 'extract-loader', 'css-loader'].

lit-css-loader gives us a nice DX of importing styles to TS views like this:

import styles from './list-view.css';

@customElement('list-view')
export class ListView extends LitElement {
  static styles = [Lumo, styles];

css-loader gives the additional benefit of resolving CSS @import "file.css"; syntax in CSS files. This is a new features that works both for TS views and also when using @CssImport() annotation in Java.

extract-loader is needed in the middle to convert the JS output of css-loader into a plain CSS string.

[Haprog]

Related to vaadin-learning-center/crm-tutorial-typescript#8

[Haprog]

I think this can be considered backwards compatible since if you were previously using imported styles in JS/TS in a string context that should still work since the CSSResult object (returned by lit-css-loader) has a toString() method that returns the plain CSS string. And it seems to work even if you still wrap the result in a unsafeCSS() call (though this change makes unsafeCSS() wrapping unnecessary so the view code looks nicer).

[Haprog]

I think this should require a minor bump (+0.1.0) since it's practically a new feature.

[platosha]

LGTM