* Add basic support for videos as types

This PR adds support for VIDEO as first-class types. In order to avoid
unnecessary costs, VIDEO outputs must implement the `VideoInput` ABC,
but their implementation details can vary. Included are two
implementations of this type which can be returned by other nodes:

* `VideoFromFile` - Created with either a path on disk (as a string) or
  a `io.BytesIO` containing the contents of a file in a supported format
  (like .mp4). This implementation won't actually load the video unless
  necessary. It will also avoid re-encoding when saving if possible.
* `VideoFromComponents` - Created from an image tensor and an optional
  audio tensor.

Currently, only h264 encoded videos in .mp4 containers are supported for
saving, but the plan is to add additional encodings/containers in the
near future (particularly .webm).

* Add optimization to avoid parsing entire video

* Improve type declarations to reduce warnings

* Make sure bytesIO objects can be read many times

* Fix a potential issue when saving long videos

* Fix incorrect type annotation

* Add a `LoadVideo` node to make testing easier

* Refactor new types out of the base comfy folder

I've created a new `comfy_api` top-level module. The intention is that
anything within this folder would be covered by semver-style versioning
that would allow custom nodes to rely on them not introducing breaking
changes.

* Fix linting issue