fix: resolve style compatibility issues for the copy button (#31)

修复复制按钮在低版本浏览器下的兼容问题。

可测试插件：[plugin-highlightjs-1.0.0-SNAPSHOT.jar.zip](https://github.com/user-attachments/files/17146103/plugin-highlightjs-1.0.0-SNAPSHOT.jar.zip)

Fixes #18 
Fixes #17

/kind bug

```release-note
修复复制按钮在低版本浏览器下的兼容问题。
```

src/main/resources/static/plugins/highlightjs-copy.css

@@ -34,6 +34,10 @@
     mask-repeat: no-repeat;
     mask-size: 1rem;
     mask-position: center center;
+    -webkit-mask: url('data:image/svg+xml;utf-8,<svg width="24" height="24" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg"><path fill-rule="evenodd" clip-rule="evenodd" d="M6 5C5.73478 5 5.48043 5.10536 5.29289 5.29289C5.10536 5.48043 5 5.73478 5 6V20C5 20.2652 5.10536 20.5196 5.29289 20.7071C5.48043 20.8946 5.73478 21 6 21H18C18.2652 21 18.5196 20.8946 18.7071 20.7071C18.8946 20.5196 19 20.2652 19 20V6C19 5.73478 18.8946 5.48043 18.7071 5.29289C18.5196 5.10536 18.2652 5 18 5H16C15.4477 5 15 4.55228 15 4C15 3.44772 15.4477 3 16 3H18C18.7956 3 19.5587 3.31607 20.1213 3.87868C20.6839 4.44129 21 5.20435 21 6V20C21 20.7957 20.6839 21.5587 20.1213 22.1213C19.5587 22.6839 18.7957 23 18 23H6C5.20435 23 4.44129 22.6839 3.87868 22.1213C3.31607 21.5587 3 20.7957 3 20V6C3 5.20435 3.31607 4.44129 3.87868 3.87868C4.44129 3.31607 5.20435 3 6 3H8C8.55228 3 9 3.44772 9 4C9 4.55228 8.55228 5 8 5H6Z" fill="black"/><path fill-rule="evenodd" clip-rule="evenodd" d="M7 3C7 1.89543 7.89543 1 9 1H15C16.1046 1 17 1.89543 17 3V5C17 6.10457 16.1046 7 15 7H9C7.89543 7 7 6.10457 7 5V3ZM15 3H9V5H15V3Z" fill="black"/></svg>');
+    -webkit-mask-repeat: no-repeat;
+    -webkit-mask-size: 1rem;
+    -webkit-mask-position: center center;
 }
 .hljs-copy-button:hover {
     background-color: color-mix(in srgb, var(--hljs-theme-color), transparent 90%);