Chore: add border zero (ElemeFE#15256)

Author: iamkun

examples/i18n/theme-editor.json

@@ -106,6 +106,7 @@
       "background-color-base": "基础背景色",
       "border-radius-base": "大圆角",
       "border-radius-small": "小圆角",
+      "border-radius-zero": "无圆角",
       "border-radius-circle": "圆形圆角",
       "box-shadow-base": "基础投影",
       "box-shadow-dark": "深色投影",

packages/theme-chalk/src/common/var.scss

@@ -86,6 +86,8 @@ $--border-radius-base: 4px !default;
 $--border-radius-small: 2px !default;
 /// borderRadius|1|Radius|0
 $--border-radius-circle: 100% !default;
+/// borderRadius|1|Radius|0
+$--border-radius-zero: 0 !default;
 
 // Box-shadow
 /// boxShadow|1|Shadow|1