⚛️ Add directives to handle providesContext and usesContext

[DAreRodz]

Closes #67

I've refactored toVdom significantly and added support for directives like in WordPress/gutenberg#44034. There are still some code I haven't moved to directives, like sourced attributes generation and inner blocks location (not sure if it is possible).

Also, I created three directives:

• One that handles type and renders the block component (if it exists).
• Another one that handles providesContext and renders a wrapper around block children.
• And the last one, which handles usesContext, that injects a context property to a block component using a HOC.

Directives are placed in src/directives.

[DAreRodz]

Here I've assumed that options.vnode would run every time h() is used. Maybe I'm wrong here. 🤔

[luisherranz]

You're not. That's the way it works.

[luisherranz]

At first glance, it looks great to me. I'm glad the current implementation of the directives is flexible enough to inject atomic functionality into the vdom. However, I was hoping we could delegate more to the wrapper component and less to the vdom internals.

But we can keep playing with this API in other PRs.

[DAreRodz]

However, I was hoping we could delegate more to the wrapper component and less to the vdom internals.

I left three things inside toVdom internals:

1. Sourced attributes computation ― it needs access to the original DOM, at least with the current implementation.
2. Block props processing (class to className, style string to object, etc.) ― I guess we can move that inside the wrapper component without much problem.
3. Inner blocks location ― this can also be moved to the wrapper component, although I found it convenient to do that during toVdom() execution. We can give this a thought in the future.

Thanks for your review @luisherranz. I'm merging this PR. 😄