FR: Allow specifying tail for truncate_* template functions

[zx8]

Describe the solution you'd like

This is a very minor nice-to-have, but would it be possible to add an optional tail to the truncate_* template functions, to indicate that truncation has occurred? Sometimes truncation occurs at a space character, and it's not obvious that there was a longer description available.

Describe alternatives you've considered

Currently, it's possible to do something like:

if(
  description.first_line().substr(0, 80).starts_with(description.first_line()),
  pad_end(80, description.first_line()),
  truncate_end(79, description.first_line()) ++ "…"
)

(After #5072 is released, I can replace this workaround with a length check)

[yuja]

fwiw, I would call the parameter as ellipsis or something, than fill_char. It shouldn't be repeated to fill the remaining space.

[zx8]

https://docs.rs/console/latest/console/fn.truncate_str.html calls it tail - I think that's probably a better fit. Title updated.