Proper support for variables in escaped values.

The following constructs are escapes: ~"..." and ~'...'. Inside, only
@{...} is a LESS variable, @foo is not! To express that, the lexer needs
to additional states ('lessstringquotes' and 'lessstringapostrophe') and
some parsing rules special to those states. The literal '~' had to be
promoted to token too. Now, instead as marking the complete LESS string
as 'less_string' token, the lexer properly tokenizes it's content which
in turn the parser can properly evaluate.

Includes a minor bug fix, t_less_variable() wasn't ever used because
t_css_ident already captured them. This part was removed from
t_css_ident becaus it's much easier to overwrite t_less_variable in lexer
states.
LESS strings can contain $MS CSS filters

.klass {
  filter: ~'alpha(opacity=2)';
}