feat(button): support custom button font-family

Author: liweijie0812

style/mobile/components/button/_index.less

@@ -120,11 +120,7 @@
   transition: all 0.3s;
   border-radius: @button-border-radius;
   outline: none;
-  /* stylelint-disable-next-line */
-  font-family:
-    PingFang SC,
-    Microsoft YaHei,
-    Arial Regular;
+  font-family: @button-font-family;
   font-weight: @button-font-weight;
   vertical-align: top;
 

style/mobile/components/button/_var.less

@@ -1,6 +1,7 @@
 @button-border-width: var(--td-button-border-width, 2px);
 @button-border-radius: var(--td-button-border-radius, @radius-default);
 @button-font-weight: var(--td-button-font-weight, 600);
+@button-font-family: var(--td-button-font-family, @font-family);
 
 @button-extra-small-font-size: var(--td-button-extra-small-font-size, @font-size-base);
 @button-small-font-size: var(--td-button-small-font-size, @font-size-base);