* Add feature flag for external Fizz runtime
Only enabled for www for now
* Add option to load Fizz runtime from external file
When unstable_externalRuntimeSrc is provided, React will inject a script
tag that points to the provided URL.
Then, instead of emitting inline scripts, the Fizz stream will emit
HTML nodes with data attributes that encode the instructions. The
external runtime will detect these with a mutation observer and
translate them into runtime commands. This part isn't implemented in
this PR, though — all this does is set up the option to use
an external runtime, and inject the script tag.
The external runtime is injected at the same time as bootstrap scripts.
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