[workspaces] Update the RN config generator for Metro's new format
[workspaces] Update the RN config generator for Metro's new format

Metro's config format changed and this commit changes our internal RN config generator to match that new format.

Test plan: built the Home bundle (with some hard coding to disable HMR for now).

exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent