Folding Range LSP support

[kv9898]

It seems that the call for folding ranges has been quite a while: posit-dev/positron#18, posit-dev/positron#2924, posit-dev/positron#3822.

I was initially only thinking about adding foldable comments, but it seems that doing this will disable the existing folding for regions and curly brackets.

As you can see in the current version of my code, I'm trying to naively iterating over the lines. Is there any internal infrastructure for analysing code lines? e.g. ignore comments and \n in the end.

Additionally, this line iteration method creates an additional problem: comment sections within functions are considered the same as those outside functions, this leads problematic handling for things like this:

# level 1 comment ####       # folding starts here
foo <- function() {
  # level 1 comment ####     # folding ends here
}

Seems that we have to handle function folding and comment folding simultaneously to solve this issue.