More robust unreachable_code_linter

[MEO265]

As soon as there is a comment after the return or stop, it is correctly not linted, but neither are the following lines.

Will produce lints:

foo  <- function() {
  return(3)
  5 + 2
}

False negative:

foo  <- function() {
  return(3)  # A comment
  5 + 2
}

The same applies to something that @MichaelChirico has already marked as a todo in the code, but I haven't found an issue about it:

lintr/tests/testthat/test-unreachable_code_linter.R

Line 181 in c8f0074

# TODO(michaelchirico): similarly, return(x); x+1 should also lint, even though

However, not only will the following expression in the same line not be marked, but also those in subsequent lines will not be marked.

False negative:

foo  <- function() {
  return(3); x + 1
  5 + 2
}

[MEO265]

I think I've already found a solution and will probably post the PR when I've finished #2123