[0.74] Create custom message handler for synthetic page

[byCedric]

Target Branch(es)

0.74

Link to commit or PR to be picked

react/react-native#43559

Description

The new "custom message handler API" for the inspector proxy in React Native 0.74 seems to work great (see #128).

Unfortunately, there was 1 oversight for the synthetic page system. This PR also instantiates the custom message handler API for the synthetic page -- the one with a stable page ID (-1).

[huntie]

Picked ✅