Support swapping "@{...}" type variables

These only occur in interpolated strings (~'...' or ~"...").
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.
Support selector interpolation

Quote from lesscss.org: "prior to LESS 1.3.1 a (~"@{name}") type of
selector was supported. Support for this will be removed in 1.4.0.".
Meanwhile, interpolated selectors look like .@{name}.

For this, the lexer needed another state. Promote literals '{}' to
tokens to be able to drop the 'istring' state when '.foo{' occurs
(rather than '.foo {').
fixup interp string
Promote literal ':' to token
Add new HTML5 'main' and 'template' tags
Enhanced hack for 'rect' DOM / function ambiguity

'rect' is a DOM element and occurs as a function. Currently, the parser
doesn't have a list of valid function names but uses css_dom elements
(and some others) to identify function calls.

Therefore the lexer got additional state tracking a while ago. This
turned out to wrongly work on constructs such as "button::-moz-$FOO,
input::-moz-$BAR", where 'input' was marked as 'css_ident' rather than
'css_dom'. This is fixed by resetting the 'in_property_decl' state
variable after a comma.
Drop unused productions
Simplifiy ~"-Some::weird(#thing)" test

I don't think it makes sense to test for that just now. The function
call parsing should be significantly enhanced before.

Changed to ~"-Some(#thing)" instead.
Ignore missing whitespace in escaped strings

Function argument lists are missing whitespaces after each comma with
the currenct escaped strings code. However, the minified version had
commas before which wasn't any better.

Likely an easy fix somewhere but shouldn't block now.