Refactor NRT rendering

[josiah-wolf-oberholtzer]

Renderables (classes implementing __render__) should now use the signature:

class SupportsRender(Protocol):
    def __render__(
        self,
        output_file_path: Optional[PathLike] = None,
        render_directory_path: Optional[PathLike] = None,
        **kwargs,
    ) -> Union[
        Callable[[], Tuple[Callable[[], Coroutine[None, None, int]], Path]],
        Tuple[Callable[[], Coroutine[None, None, int]], Path],
    ]:
        ...

... returning a 2-tuple of:

• a zero-argument callable that returns a coroutine returning an exit code
• and the path to the anticipated output

... or a zero-argument callable that results in that 2-tuple (which makes rendering online buffers in Supriya's Sphinx documentation possible given that the buffer is offline by the time the HTML is rendered out).

While this is a kind of tricky-interface, it really simplifies the internals of Session-rendering. Sessions can now determine the expected output paths of all renderables in their dependency graph before anything is rendered, and can perform rendering asynchronously.

Sessions can now also be rendered with their output suppressed, e.g. sent to /dev/null (or NUL on Windows), which greatly simplifies doing offline analysis where the only desired output is from a manually-written buffer.