feat (advanced): split next-markdown into smaller modules

[frouo]

Split the lib in several parts

• npm install @nextmarkdown/core : contains everything but the markdownToHtml logic (commonjs lib)
• npm install @nextmarkdown/markdown-to-html : will contain all the logic to convert markdown to html with remark / rehype plugins dependencies (esm module lib because rehype-stringify is ESM only).
• npm install @nextmarkdown/mdx : will contain the next-mdx-remote logic.

Finally npm install next-markdown would install, by default, both @nextmarkdown/core and @nextmarkdown/markdown-to-html so it is really easy to get started with next-mardown + covers 80% of the usage cases.

[edvincandon]

option 2 🎉