Replace hacky `.progress` bar with official progressr machinery

See discussion in #305, in particular the ideal solution would be https://github.com/futureverse/furrr/pull/305#issuecomment-3825135653 where `progressr::handlers(global = TRUE)` could be set _as the default_ during progressr's package startup process.

This would mean that `future_map(x, fn, .progress = TRUE)` that just signals progression conditions as #305 does would show a progress bar to users by default without them having to do anything else (maintaining nice backwards compat with old furrr behavior).