Fix LESS syntax highlighting

[stevepiercy]

With the following code sample, where an @import statement is placed within a selector:

.loadThemeFonts() {
  @import "./typography.css";
}

...the Pygments LESS lexer marks it as invalid. Whereas in the Less Sandbox there is no error.

I reported the issue in Pygments pygments/pygments#2606, with a possible fix.