Add mix-blend-mode CSS property. Closes codemirror#5625

Author: curran

mode/css/css.js

@@ -501,7 +501,7 @@ CodeMirror.defineMode("css", function(config, parserConfig) {
     "margin-bottom", "margin-left", "margin-right", "margin-top",
     "marks", "marquee-direction", "marquee-loop",
     "marquee-play-count", "marquee-speed", "marquee-style", "max-height",
-    "max-width", "min-height", "min-width", "move-to", "nav-down", "nav-index",
+    "max-width", "min-height", "min-width", "mix-blend-mode", "move-to", "nav-down", "nav-index",
     "nav-left", "nav-right", "nav-up", "object-fit", "object-position",
     "opacity", "order", "orphans", "outline",
     "outline-color", "outline-offset", "outline-style", "outline-width",