Add method to convert a parsed metastring back into a string

[michaeloliverx]

The title basically explains it, I am writing a remark plugin that runs prior to remark-shiki-twoslash and I would like to parse the metastring, remove a key and use it for something unrelated and replace the metastring with that key removed.

This code can live in userland but I think it would be useful to be bundled here.

[JSerZANP]

+1 for this