fix(parser): ! in neg. operators mistranslated to dots

[koubas]

Internally, dots were temporarily translated to exclamation marks and back.
When the exclamation marks were replaced back to dots, even the literal ones from the source, like the ones in negated operators, were unintentionally replaced:

      + expected - actual

       {
         "server": {
      -    "if ($var .= foo)": {
      +    "if ($var != foo)": {
             "return": "301 https://$host$request_uri"
           }
         }
       }

I fixed that by choosing a less ambiguous replacement token (<_DOT_TOKEN_>) instead of the exclamation mark.