chore(css): Replace CSS Charsets group with CSS Syntax group (#883)

Co-authored-by: Brian Smith <[email protected]>

Author: skyclouds2001

css/at-rules.json

@@ -2,7 +2,7 @@
   "@charset": {
     "syntax": "@charset \"<charset>\";",
     "groups": [
-      "CSS Charsets"
+      "CSS Syntax"
     ],
     "status": "standard",
     "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/@charset"

css/definitions.json

@@ -12,7 +12,6 @@
       "CSS Box Model",
       "CSS Box Sizing",
       "CSS Cascading and Inheritance",
-      "CSS Charsets",
       "CSS Color",
       "CSS Columns",
       "CSS Conditional Rules",
@@ -48,6 +47,7 @@
       "CSS Shadow Parts",
       "CSS Shapes",
       "CSS Speech",
+      "CSS Syntax",
       "CSS Table",
       "CSS Text",
       "CSS Text Decoration",