CocoaPods: Split React-jsc out of React-jsi (#35031)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/35031

The React-jsi Pod was serving two purposes: building JSI, and configuring JavaScriptCore as the JS engine.

By splitting the React-jsi Pod into React-jsi and React-jsi, we can start working towards de-coupling the JSI dependency from any particular JS engine.

Pods that depended on React-jsi, now depend on React-jsi and React-jsc.
One exception to this is React-hermes, which is only installed when Hermes is enabled, and thus does not require JavaScriptCore.
Upcoming commits should take care of removing the React-jsc dependency when Hermes is enabled, but it is out of scope for this commit.

Changelog:
[iOS][Changed] - The JSC Runtime is now provided by the React-jsc Pod instead of React-jsi. Libraries that declared a dependency on React-jsi in order to specifically create a JSC runtime (`makeJSCRuntime()`) will need to add React-jsc to their dependencies.

Reviewed By: dmytrorykun

Differential Revision: D40442603

fbshipit-source-id: b9b21146b9deb401f80cfef76a87c9867754a953