CSSLint/CodeMirror: background-position-x and -y

These have been used for a long time in the real world and will finally get standardized by the upcoming CSS Backgrounds and Borders Module L4.

fixes #397

edit/codemirror-default.js

@@ -102,6 +102,10 @@
   }
 
   Object.assign(CodeMirror.mimeModes['text/css'].propertyKeywords, {
+    // CSS Backgrounds and Borders Module L4
+    'background-position-x': true,
+    'background-position-y': true,
+
     // CSS Logical Properties and Values L1
     'block-size': true,
     'border-block-color': true,

vendor-overwrites/csslint/parserlib.js

@@ -161,7 +161,9 @@ self.parserlib = (() => {
     'background-color':        '<color>',
     'background-image':        '<bg-image>#',
     'background-origin':       '<box>#',
-    'background-position':     '<bg-position>',
+    'background-position':     '<bg-position>#',
+    'background-position-x':   '[ center | [ left | right ]? <length-percentage>? ]#',
+    'background-position-y':   '[ center | [ top | bottom ]? <length-percentage>? ]#',
     'background-repeat':       '<repeat-style>#',
     'background-size':         '<bg-size>#',
     'baseline-shift':          'baseline | sub | super | <percentage> | <length>',