chore[react-devtools]: add global for native and use it to fork backend implementation (#30533)

Adding `__IS_NATIVE__` global, which will be used for forking backend
implementation. Will only be set to `true` for `react-devtools-core`
package, which is used by `react-native`.

Ideally, we should name it `react-devtools-native`, and keep
`react-devtools-core` as host-agnostic.

With this change, the next release of `react-devtools-core` should
append component stack as Error object, not as string, and should add
`(<anonymous>)` suffix to component stack frames.