Enable progressr if output is connected to a TTY - a slight improvement to interactive mode

The default is to enable progress handler per:

```r
enable = getOption("progressr.enable", interactive())
```

The rationale is that you'll get progress updates when running R interactively, but not in batch mode (e.g. `Rscript main.R`).

However, it might be more natural to condition it on whether the output is connected to a TTY, e.g. `isatty(stdout())` or `isatty(stderr())`. However, does this logic apply to all progress handlers, or just those outputting to the terminal, or ...

Start by reviewing the default enable expression using interactive() and investigate R's isatty(stdout()) and isatty(stderr()) behavior. Determine whether TTY-based enabling should apply to every progress handler or only terminal-output handlers, then define the expected behavior for interactive and batch execution before adding coverage.

Written by the indexing model from the issue text.