Moves the `Drop` logic for `GPURenderBundle` into a dedicated helper
struct, `DroppableGPURenderBundle`. This change ensures that
`GPURenderBundle` itself does not directly implement `Drop`, aligning
with the policy to prevent direct `Drop` implementations on DOM types.
Removes the `allowDropImpl` configuration for `GPURenderBundle` from
script bindings.
Testing: WebGPU tests cover all related cases
Fixes: Partially #26488
Signed-off-by: Domenico Rizzo <domenico.rizzo@gmail.com>