fix: Add CSS for hljs-emphasis and hljs-strong in playground

Author: dereuromark

docs/.vitepress/theme/custom.css

@@ -225,12 +225,19 @@
 .djot-playground .preview .hljs-title,
 .djot-playground .preview .hljs-section,
 .djot-playground .preview .hljs-doctag,
-.djot-playground .preview .hljs-name,
-.djot-playground .preview .hljs-strong {
+.djot-playground .preview .hljs-name {
   color: var(--vp-c-brand-1);
   font-weight: 600;
 }
 
+.djot-playground .preview .hljs-strong {
+  font-weight: 700;
+}
+
+.djot-playground .preview .hljs-emphasis {
+  font-style: italic;
+}
+
 .djot-playground .preview .hljs-string,
 .djot-playground .preview .hljs-attr {
   color: #22863a;