Improve CSS Syntax highlight (support for SCSS)

[eenaree]

Changed

SCSS Syntax highlight

• comment (Dark theme)
• mixin at-rule and mixin's name
• interpolation
• placeholder selector
• suffix for parent selector
• In for, each, media at rules, keyword.control.operator ex) each - in / for - from, to / media - only, and, etc...
• key of map
• variables

Common Syntax highlight

• In Light theme, supported property value
• language code of pseudo class lang ex) en, fr, de
• constant keyword of pseudo class starting with :nth ex) even, odd
• attribute name of attribute selector
• unquoted attribute name of attribute selector
• parameter value of css value functions
• css @keyframes identifier
• make vendor prefix property name the same as normal property name
• differentiate color when defining css custom property and when using them (only In CSS)
• If VSCode editor's textmate is invalid, make token red color