by @patarapolw
* allow manual init and destroy for use with Nuxt
* fix onDestroy-related methods
* avoid global scope, and use function scope instead
* add createInstance function to window.REMARK42
* 1. allow DOMNode to be put in remark_config 2. check remark_config before try to attach node
* move createInstance function outside
* update embed.ts
* avoid ?.
* 📚 Docs: doc on how to make it work with SPAs
* 📚 Docs: fix spa.md to be more flexible
* ✨ Feat: add REMARK42::ready event
* remove nuxt-specific terminologies
* tell MutationObserver to disconnect on destroy
* update docs/spa.md