Feat: add support for @import CSS-rule

[dgorbash]

Added support for @import css-rule. It can help with inlining imported stylesheets such as Google Fonts.

The feature is disabled by default but can be enabled using the imports: true option.

Example:

css(
  {
    fileContent: '@import url('https://fonts.googleapis.com/css2?family=Open+Sans&display=swap')',
    imports: true
  }, 
  (err, content) => console.log(content)
)

Output:

@font-face {
  font-family: 'Open Sans';
  font-style: normal;
  font-weight: 400;
  font-stretch: 100%;
  font-display: swap;
  src: url("data:font/woff2;base64,...) format('woff2');
}
...

Without imports: true the output will be:

@import url('data::text/css; charset=utf-8;base64,...');

[jrit]

Thank you for the PR, could you please add or update a test that exercises the change?