feat(css): Update syntax for *-linear-gradient() (#948)

* feat(css): Update syntax for `linear-gradient()`

* update

Author: skyclouds2001

css/functions.json

@@ -360,7 +360,7 @@
     "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/easing-function/linear"
   },
   "linear-gradient()": {
-    "syntax": "linear-gradient( [ <angle> | to <side-or-corner> ]? , <color-stop-list> )",
+    "syntax": "linear-gradient( [ <linear-gradient-syntax> ] )",
     "groups": [
       "CSS Images"
     ],
@@ -536,7 +536,7 @@
     "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/gradient/repeating-conic-gradient"
   },
   "repeating-linear-gradient()": {
-    "syntax": "repeating-linear-gradient( [ <angle> | to <side-or-corner> ]? , <color-stop-list> )",
+    "syntax": "repeating-linear-gradient( [ <linear-gradient-syntax> ] )",
     "groups": [
       "CSS Images"
     ],

css/syntaxes.json

@@ -537,7 +537,10 @@
     "syntax": "linear | <linear()>"
   },
   "linear-gradient()": {
-    "syntax": "linear-gradient( [ [ <angle> | to <side-or-corner> ] || <color-interpolation-method> ]? , <color-stop-list> )"
+    "syntax": "linear-gradient( [ <linear-gradient-syntax> ] )"
+  },
+  "linear-gradient-syntax": {
+    "syntax": "[ [ <angle> | <zero> | to <side-or-corner> ] || <color-interpolation-method> ]? , <color-stop-list>"
   },
   "log()": {
     "syntax": "log( <calc-sum>, <calc-sum>? )"
@@ -798,7 +801,7 @@
     "syntax": "repeating-conic-gradient( [ <conic-gradient-syntax> ] )"
   },
   "repeating-linear-gradient()": {
-    "syntax": "repeating-linear-gradient( [ <angle> | to <side-or-corner> ]? , <color-stop-list> )"
+    "syntax": "repeating-linear-gradient( [ <linear-gradient-syntax> ] )"
   },
   "repeating-radial-gradient()": {
     "syntax": "repeating-radial-gradient( [ <ending-shape> || <size> ]? [ at <position> ]? , <color-stop-list> )"