Dart Sass 1.0.0-alpha.8

• Add the content-exists() function.
• Support interpolation in loud comments.
• Fix a bug where even valid semicolons and exclamation marks in custom property values were disallowed.
• Disallow invalid function names.
• Disallow extending across media queries.
• Properly parse whitespace after ... in argument declaration lists.
• Support terse mixin syntax in the indented syntax.
• Fix @at-root query parsing.
• Support special functions in @-moz-document.
• Support ... after a digit.
• Fix some bugs when treating a map as a list of pairs.