Define NODE_ENV to use the production build of React on slatejs.org

[schneidmaster]

Is this adding or improving a feature or fixing a bug?

Improvement

What's the new behavior?

Defines process.env.NODE_ENV through webpack's DefinePlugin. Defining this value causes us to use the production build of React on slatejs.org, which has better performance (and is thus a better representation of how Slate works, especially with large documents).

Specifically -- defining process.env.NODE_ENV makes code paths unreachable within the core React code (by producing, in essence, true === false conditionals), and then Uglify strips out those unreachable code paths, resulting in React not running development code and being faster.

How does this change work?

See previous section

Have you checked that...?

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn prettier.)
• The relevant examples still work. (Run examples with yarn watch.)

Does this fix any issues or need any specific reviewers?

Reviewers: @ianstormtaylor

[ianstormtaylor]

Beautiful, thanks @schneidmaster!