Context API

[leopiccionia]

This PR includes basic support to React-like Context API, including:

• createContext
• useContext
• Context.Provider & Context.Consumer

This PR doesn't include:

• Support for the (undocumented?) second parameter of createContext
• Support for Class.contextType
• Support for displayName¹

At a quick look at React Aria source, I couldn't locate many references to Provider and Consumer as components. If you don't consider them useful, I could remove it.

I've implemented those functions using analogous provide/inject features present in Vue. The implementation supposes that contexts are created and consumed inside Vue.

¹ It could be investigated in future. I don't know if Vue allows component's name as a getter.

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I couldn't apply the patches and run the demo, so I couldn't test them against the React Aria codebase; therefore, I'm marking the PR as "draft".

For a running demo, see https://codepen.io/leopiccionia/pen/JjXzwON.

[devongovett]

Nice! Thanks for sending a PR!