Respect `browserNLdisabled` in our custom parse/eval loop so `Enter` advances the debugger

[DavisVaughan]

posit-dev/positron#11440

• debugonce(dplyr:::mutate.data.frame)
• dplyr::mutate(mtcars, x = 1)
• Press Enter while in a blank console, notice how the debugger does not advance

@juliasilge has confirmed that this used to advance the debugger. It is very likely that #960 is what changed this behavior.

In RStudio, and following the intention of browserNLdisabled, if you follow the instructions above then Enter in a blank console is supposed to count as n when in debug() mode.
https://github.com/wch/r-source/blob/ca917f5f467e9dd6ee4b867deaabff1f78a8b3e2/src/main/main.c#L232-L237

It seems like we are missing out on this branch, or maybe we have to implement it manually. It seems like a nice feature to ensure we support.

After manually adding support for the Enter feature, we'd also want to make sure that options(browserNLdisabled = TRUE) turns this back off