Make public map_from in json2

[esquerbatua]

module x.json2

It's useful to make public as map function to transform any structure to modify data in the fly and transform the map into json directly

[spytheman]

The CI failure is for vlib/x/json2/json2.v:432:0:pub fn map_from[T](t T) map[string]Any , since it is now public, it needs to be documented.
Please add a short comment above it, starting with: // map_from to fix it.

[esquerbatua]

Add docs to map_from