feat(allocator)!: make `Allocator::cursor_ptr` and `data_end_ptr` private (#26081)

`Allocator::cursor_ptr` and `Allocator::data_end_ptr` were only exposed for `oxc_linter` to use for some dodgy pointer laundering. #26077 removed that code, so these methods no longer need to be exposed.

Remove them from public API - it's not ideal to be exposing the internals of the allocator to user code. Both are still used within `oxc_allocator` crate, so they're not removed entirely, but become `pub(crate)`.