Support partial rendering for e. g. Quart.async_generator()

[embeddedc]

Example:

@app.get("/table")
async def stream_html_table():
    async def async_generator():
        t = table()
        with t:
            with thead():
                with tr():
                    th("Row 1");  th("Row2");  th("Row3")
            with tbody():
                yield t.render_without_closing_tags()
            for r in range(rows):
                tr = tr()
                with tr:
                    (a, b, c) = await calculate_next_row()
                    td(a); td(b); td(c)
                    yield tr.render()
            yield t.render_closing_tags()

    return async_generator(), 200