`console_width()` should use a fixed width in .Rmd

[gaborcsardi]

Or maybe use getOption("width"), but at least it should use the terminal width, even if the console is a proper terminal.

From @DavisVaughan

EDIT: this seems to be a problem in Positron only, because Positron has

``` r
options(width = 100)
isatty(stdin())
```

    ## [1] TRUE

``` r
cli::is_ansi_tty()
```

    ## [1] TRUE