Lines inside fenced code blocks that start with an octothorpe are interpreted as headers and break folding

Nobody has claimed this yet.

This looks very much along the lines of #1. Possibly related to #6 also (but I kinda doubt it).

Using vim-markdown-folding with vim-plug, on 3f35acfb753cc9ea22182400b075c5b6e896ad71. I'm observing a problem when editing [this file](https://raw.githubusercontent.com/rpdelaney/iris-setup/a73f04ab1d0d0cc587d40893af2bf48c7eb81ba7/PRE_OS.md) I'm working on.

Looks like this:
```
# Pre-OS prep [17 lines]-----------------------------------------------------------------------------------
## Installation media [13 lines]-----------------------------------------------------------------------------------
## Create partitions [4 lines]------------------------------------------------------------------------------------
# gdisk /dev/nvme0n1 [7 lines]------------------------------------------------------------------------------------
## Create LUKS container [4 lines]------------------------------------------------------------------------------------
# cryptsetup luksFormat --type luks1 --use-random --hash whirlpool --iter-time 5000 /dev/nvme0n1p2 [4 lines]---------------------------
## O/S Filesystem [7 lines]------------------------------------------------------------------------------------
### Create and mount LVM volumes [6 lines]------------------------------------------------------------------------------------
# vgcreate volgroup0 /dev/mapper/cryptlvm
# lvcreate volgroup0 -L 32G -n lvswap
# lvcreate volgroup0 -l 100%FREE -n lvroot [5 lines]-----------------------------------------------------------------------------------
### Create btrfs filesystem [4 lines]------------------------------------------------------------------------------------
# mkfs.btrfs -L root /dev/volgroup0/lvroot
# mkswap -L swap /dev/volgroup0/lvswap
# swapon -L swap [56 lines]-----------------------------------------------------------------------------------
## O/S [4 lines]------------------------------------------------------------------------------------
```

I would expect something more like this (maybe with different counts of folded lines):
```
# Pre-OS prep [17 lines]-----------------------------------------------------------------------------------
## Installation media [13 lines]-----------------------------------------------------------------------------------
## Create partitions [4 lines]------------------------------------------------------------------------------------
## Create LUKS container [4 lines]------------------------------------------------------------------------------------
## O/S Filesystem [7 lines]------------------------------------------------------------------------------------
### Create and mount LVM volumes [6 lines]------------------------------------------------------------------------------------
### Create btrfs filesystem [4 lines]------------------------------------------------------------------------------------
## O/S [4 lines]------------------------------------------------------------------------------------
```

Reproduce the folding behavior with vim-markdown-folding using the linked PRE_OS.md example and the reported commit. Start by tracing how fenced code blocks and lines beginning with octothorpes are classified, then verify that command lines inside the fence no longer create folds while Markdown headings still fold as expected.

Written by the indexing model from the issue text.