fix: pass in generic types and export Specs

[itsramiel]

Summary

The nitrogen page is a step by step guide on running Nitro Codegen but following as it is now leads user to errors.

When defining the Spec, HybridObject requires a generic parameter or else the user would get:

        ❌  Failed to generate spec for RNFilePicker! Error: RNFilePicker does not properly extend HybridObject<T> - HybridObject does not have a single generic type argument for platform spec languages.
    ❌  No specs found in RNFilePicker.nitro.ts!

So this pr adds the generic type for guide completeness and exports the Spec since it is used in step 6 which I also made obvious in that step that this is the spec defined in .nitro.ts file.

Also the classes are missing the memorySize and hybridContext(for ios) for completeness. I copy pasted them, only to get a cryptic error in the console till i opened xcode. C++ needs to be updated too for memorySize? Not sure

[mrousavy]

looks good, but I have a few suggestions to keep it a bit cleaner - less text is more readable.