Improve folding

[renkun-ken]

Close #316

This PR make folding range provider only create folding ranges for multi-line expressions with (), [] and {}, and the folding range end is a line above the closing bracket so when the range is folded the closing bracket could be seen. Now the folding ranges are consistent with behavior of most other language servers.

if (x > 0) {
  # hello
  # hello2
  x + 1
} else if (x < -1) {
  x - 1
} else {
  x
}

Before:

After:

And multi-line expressions not enclosed in a pair of brackets are no longer foldable. For example, a pipeline expression could no longer be folded as it is not enclosed in brackets.

kable(dt, format = "html", caption = "Demo Table") %>%
  kable_styling(bootstrap_options = "striped",
    full_width = FALSE) %>%
  add_header_above(c(" ", "Group 1" = 2, "Group 2[note]" = 2)) %>%
  add_footnote(c("table footnote"))

[randy3k]

Please just merge it once you feel good about it. :)