CSS strings can contain variables too

Ordinary CSS strings may contain @{...}-style variables. Here are some
examples:

    content: "@{breadcrumb-separator}\00a0";
    background-image: url("@{file-2x}");
    .col(@index + 1, ~"@{list}, @{item}");
Add t_tilde token for '~'

To be used inside general_sibling_selector (e.g. 'a ~ b { ... }')
instead of the literal '~'. Necessary since we had to remove the literal
from the lexer in order to tokenize ~"..." and ~'...'.
Bring back support for @@name variables
Properly name escapes and interpolated strings

Escapes are seperate to interpolated strings.