fix: style tag transform leak

Author: antfu

packages/slidev/node/plugins/markdown.ts

@@ -121,7 +121,7 @@ export function transformPageCSS(md: string, id: string) {
   const result = md.replace(
     /(\n<style[^>]*?>)([\s\S]+?)(<\/style>)/g,
     (full, start, css, end) => {
-      const index = md.search(full)
+      const index = md.indexOf(full)
       // don't replace `<style>` inside code blocks, #101
       if (index < 0 || codeblocks.some(([s, e]) => s <= index && index <= e))
         return full