enable auto rerender

[kaizhang16]

📑 Summary

Enable auto rerender when pnpm run docs:dev. And other projects may also be inspired about how to integrate mermaid into vitepress.

📏 Design Decisions

According to https://vuejs.org/guide/essentials/watchers.html, watch track props, and will call renderChar when props changes.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added unit/e2e tests (if appropriate)
• 📓 have added documentation (if appropriate)
• 🔖 targeted develop branch

[sidharthv96]

Thank you for the PR @kaizhang16, but we already have file watching in docs:dev which re-renders the diagram wh`n the file changes.

Please reopen if you faced a scenario where editing files in packages/mermaid/src/docs didn't change the diagram automatically.

NB: Editing <root>/docs will not change the diagrams, but that is not a problem as packages/mermaid/src/docs is where the editing should be done.