Set React Client to PRODUCTION by default

[Archmonger]

Old Behavior

Currently, the react client is set to development by default.

New Behavior

Build the react client in production mode by default, or intelligently select a client based on IDOM_DEBUG_MODE

Implementation Details

Change the const { PRODUCTION } = process.env; to default to True

Consider a method where this can be done at run-time.

See related issue reactive-python/reactpy#346

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