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.

src/App.zig

@@ -524,6 +524,16 @@ fn hasSurface(self: *const App, surface: *const Surface) bool {
     return false;
 }
 
+/// Search for a surface by a 64 bit unique ID.
+pub fn findSurfaceByID(self: *const App, id: u64) ?*Surface {
+    for (self.surfaces.items) |v| {
+        const surface: *Surface = v.core();
+        if (surface.id == id) return surface;
+    }
+
+    return null;
+}
+
 fn hasRtSurface(self: *const App, surface: *apprt.Surface) bool {
     for (self.surfaces.items) |v| {
         if (v == surface) return true;