Upgrade js-yaml to v5.1.0 and migrate to Vitest from Jest (#215)

* Add js-yaml ^5.1.0 devDependency; markdown-it already at 14.2.0

* Migrate Jest to Vitest; remove @babel/core override

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

eslint.config.js

@@ -9,7 +9,7 @@ export default [
       globals: {
         ...globals.es6,
         ...globals.node,
-        ...globals.jest,
+        ...globals.vitest,
       },
     },
     files: ["**/*.{js,mjs,cjs,jsx,mjsx,ts,tsx,mtsx}"],
@@ -20,7 +20,7 @@ export default [
       "import/no-unresolved": [
         "error",
         {
-          ignore: ["^markdownlint/.+"],
+          ignore: ["^markdownlint/.+", "^vitest/.+"],
         },
       ],
     },