Avoid parsing LESS, PostCSS too

[hmsk]

Other than Sass/SCSS, we may face parsing errors with cssExtract.extractClasses(cssCode); in processStyle (stylePart, filePath) if you give code which is invalid as pure CSS. css module can't parse.

like

.testPostcCSS {
  body {
    @media screen {
      background-color: red;
    }
  }
}

This syntax is allowed for almost CSS preprocessors. Anyway, some other syntaxes may make an exception with css module. So, I just added exts to ignore compilation.

[eddyerburgh]

Thanks 😀