core: add 64 bit unique ID to every core surface

- Expose that ID as the environment variable GHOSTTY_SURFACE_ID to
  processes running in Ghostty surfaces.
- Add a function to the core app to search for surfaces by ID.
- ID is randomly generated, it has no other meaning other than as a
  unique identifier for the surface. The ID also cannot be zero as that
  is used to indicate a null ID in some situations.