[compiler] Don't leak global __DEV__ type

[eps1lon]

babel-plugin-react-compiler is not providing a global __DEV__ so it shouldn't declare its type. Whatever is providing this global should declare the types.

This prevents including __DEV__ in autocomplete when that variable may never be available.
This was especially problematic if you had an existing __DEV__ declared because then babel-plugin-react-compiler would fail since you can't re-declare block scoped variables.

Diff for dist/index.d.ts

 declare global {
-  let __DEV__: boolean | null | undefined;
 }