feat: support CJK & ANS spacing (#12)

lib/_heading.scss

@@ -23,19 +23,16 @@
     margin-block-end: $std-block-unit;
     font-size: $font-size-h1;
     line-height: $line-height-size-h1;
-    letter-spacing: $font-size-h1 * 0.05;
   }
 
   h2 {
     font-size: $font-size-h2;
     line-height: $line-height-size-h2;
-    letter-spacing: $font-size-h2 * 0.05;
   }
 
   h3 {
     font-size: $font-size-h3;
     line-height: $line-height-size-h3;
-    letter-spacing: $font-size-h3 * 0.05;
   }
 
   h4 {
@@ -53,6 +50,18 @@
     line-height: $line-height-size-h6;
   }
 
+  h1,
+  h2,
+  h3 {
+    // 中文大标题增加微小文字间距
+    letter-spacing: 0.05em;
+
+    // 非中文时不加间距
+    @include non-cjk-block {
+      letter-spacing: 0;
+    }
+  }
+
   // 压缩两个标题之间的间距
   h1 + h2,
   h2 + h3,