refactor: styles of copy button (#14)

优化复制按钮的显示方式，修复出现多个滚动条的问题。

/kind bug

Fixes #13 

before:

<img width="500" alt="image" src="https://github.com/halo-sigs/plugin-highlightjs/assets/21301288/7dd6b773-af3c-4857-9d8a-cdb29b594393">

<img width="501" alt="image" src="https://github.com/halo-sigs/plugin-highlightjs/assets/21301288/517ccbc8-38b6-44b4-86ec-285edacfe139">

after:

<img width="503" alt="image" src="https://github.com/halo-sigs/plugin-highlightjs/assets/21301288/658c9d0c-b422-491b-8e46-7abddd5c16da">

```release-note
优化复制按钮的显示方式，修复出现多个滚动条的问题。

```

src/main/resources/static/plugins/override.css

@@ -1,3 +1,22 @@
 .hljs-copy-button[data-copied="true"] {
   padding: 0 0.5em;
 }
+
+.hljs-copy-button {
+  transform: translateX(0) !important;
+  display: none;
+}
+
+.hljs-copy-wrapper:hover .hljs-copy-button,
+.hljs-copy-button:focus {
+  display: block;
+}
+
+.hljs-copy-button:hover {
+  background-color: var(--hljs-theme-background) !important;
+  filter: brightness(1.1);
+}
+
+.hljs-copy-alert {
+  display: none;
+}