Spike: move the napi host and addon loading into React Native core #495

Prove the RFC in #447: an app with no react-native-node-api native host can load an addon linked against the Hermes stub from #494, with core providing the napi host and addon loading.

Steps

• On a branch of a react/react-native fork, move the napi host and addon loading into core. Android first.
• Build an app from that source, without our native host, with an addon linked against the stub.
• Check that the addon loads, its calls land in Hermes, and the relevant tests pass.
• Publish the branch as a draft PR on the fork, so the RFC has a concrete diff to point at.

What this should settle

• What has to move. Something must load the addon as well as host it. Today that path is require("./addon.node") → Babel transform → requireNodeAddon(). Core needs one primitive for this, while the Babel plugin, link and the naming scheme may be able to stay here.
• Whether hermes_napi.h can stay internal. It declares embedder API such as hermes_napi_create_env and hermes_run_script, so it should be a private dependency of core's napi host, not a public header. Addons use node-api-headers.

What the RFC asks for

1. Hermes builds used by React Native export Node-API (Remove Hermes vendoring once React Native ships a Hermes with Node-API #412). static_h already has API/napi, but it isn't on main or in any release. This spike builds on our vendored Hermes in the meantime.
2. The library exporting it keeps a stable name, or React Native declares a supported one. 0.82 renamed libhermes.so to libhermesvm.so, and another rename would break published addons.

node-api-headers defines the interface, so a mismatch with it is a Hermes bug and needs no separate ask.

Depends on the first step of #494.