niso_app.h: apps in any language

From crates/niso-app/include/niso_app.h, a header of its own outside niso_runtime.h.

Functions: niso_app_main

How a host runs a niso app written in any language.

An app is a library that exports niso_app_main and does everything else through niso.h. The host creates the DOM and sets it up as for any page (viewport, niso_dom_set_control_size, niso_dom_set_button_style), then calls niso_app_main once, before the first frame. The app builds its document there: elements, a