Add CodSpeed for continuous performance benchmarking

- Downgrade vitest from v4 to v3.2.4 for CodSpeed compatibility
- Add @codspeed/vitest-plugin v5.0.1
- Configure vitest with CodSpeed plugin
- Add GitHub Actions workflow for automated benchmarking
- Add CodSpeed badge to README

Author: codspeed-hq[bot]

package.json

@@ -1,7 +1,7 @@
 {
   "name": "@markdown-exit/monorepo",
   "type": "module",
-  "version": "1.0.0-beta.6",
+  "version": "1.0.0-beta.7",
   "private": true,
   "packageManager": "pnpm@10.26.2",
   "license": "MIT",

packages/markdown-exit/package.json

@@ -1,7 +1,7 @@
 {
   "name": "markdown-exit",
   "type": "module",
-  "version": "1.0.0-beta.6",
+  "version": "1.0.0-beta.7",
   "description": "A TypeScript rewrite of markdown-it with enhancements",
   "license": "MIT",
   "homepage": "https://github.com/serkodev/markdown-exit#readme",

packages/testgen/package.json

@@ -1,7 +1,7 @@
 {
   "name": "@markdown-exit/testgen",
   "type": "module",
-  "version": "1.0.0-beta.6",
+  "version": "1.0.0-beta.7",
   "private": true,
   "license": "MIT",
   "main": "./src/index.ts",