Support folding ranges for comments

[renkun-ken]

This PR adds support of folding ranges for comments so that consecutive lines of comments that start with the same column can be folded.

# comment range 1
# comment range 1
a <- 1

# comment range 2
# comment range 2
b <- 2

# not comment range
c <- 3

f <- function(x, y) {
  # comment range 3
  # comment range 3
  x + y
}

The comments in the same folding ranges could be folded.

[randy3k]

LGTM