Use the modern JSX transform (closes #123)

[rome-user]

With the new transform, the $ macro is technically no longer dependent on React. This possibly opens a path to Preact support in Helix.

More importantly, we obtain marginally faster performance. React is able to make optimizations that it otherwise can't with React.createElement().

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Backwards compatibility While the new JSX transform was introduced in React 17, it has been backported to React 16.14.0, React 15.7.0, and React 0.14.10. This PR should not introduce any surprises to current users of Helix, unless they use outdated versions of React 16, 15, or 0.14.

Pros

• Marginally faster performance.
• Benefitting from future React optimizations

Cons

• Users on versions of React below 0.14.10 will no longer be able to use Helix. Users on outdated versions of React 0.14, 15, or 16 will no longer be able to use Helix.
• The logic of the $ and $d macros are slightly more complicated. The logic of the $ function is also more complicated. Unit tests are provided to ensure the output of $ matches the modern JSX transform, but these tests may need maintenance.

[lilactown]

I'll review this week. Thank you so much!

[lilactown]

Still on my list. @rome-user could you resolve the conflicts on this branch?

[rome-user]

Sure. I'll be able to do that over the next few days