Incorrect handling of nested comments

[belamenso]

Per https://nim-lang.org/docs/manual.html#lexical-analysis-multiline-comments Nim supports nesting multiline comments.

This code, when ran outputs only 3:

#[
  #[
  echo "1"
  ]#
echo "2"
]#

echo "3"

NVIM v0.4.3, nim.vim commit ea3bd85, Kubuntu 18.04