Add support for lit-css

[web-padawan]

lit-css is a new tool to distribute styles via ES modules.
The syntax it uses is exactly the same as used by astroturf added by #37:

import { css } from 'lit-css';

export default css`
  .table {
    /* my default table styles */
  }
`;

Adding the support would allow us to process the CSS in tagged literals used by this library.

This is especially important, as lit-css does not extract styles into .css files, unlike most of tools using similar syntax do. Its output is used as a content for <style> tags used e.g. by lit-html.

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This change is 

[codecov]

Codecov Report

Merging #38 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #38   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         403    403           
=====================================
  Hits          403    403

Impacted Files	Coverage Δ
extract.js	100% <ø> (ø)	⬆️

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[web-padawan]

Tested and confirmed to work in web-padawan/lit-css-sandbox@22058b1