perf: improve performance for the `includeExplanation` option

[thetarnav]

This improves the performance for using codeToTokens with includeExplanation option enabled.

I'm working with a file that is 3k lines long, and getting the tokens with explanations was taking way too much time, around 7 to 9 seconds.

Turns out that the explainThemeScope function was redoing a decent amount of work for each token, for each scope, and for each setting, without depending on any of those. So it would be calculated once and reused.

Now I get tokens for that file in ~1.5s

Probably could be improved more, but I didn't want to change any logic.

[netlify[bot]]

✅ Deploy Preview for shiki-next ready!

Name	Link
🔨 Latest commit	19c59a98611278b1cfe3f3210c038d751b510fe3
🔍 Latest deploy log	https://app.netlify.com/sites/shiki-next/deploys/666dd6731ef92d0008b490e2
😎 Deploy Preview	https://deploy-preview-701--shiki-next.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[netlify[bot]]

✅ Deploy Preview for shiki-matsu ready!

Name	Link
🔨 Latest commit	19c59a98611278b1cfe3f3210c038d751b510fe3
🔍 Latest deploy log	https://app.netlify.com/sites/shiki-matsu/deploys/666dd673148c0e0008b5f17f
😎 Deploy Preview	https://deploy-preview-701--shiki-matsu.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.