Fix [...] directories not showing children

Description of Changes

Correctly escape directories named [<dir_name>] on Windows. Per :help wildcards:

To avoid the special meaning of the wildcards prepend a backslash.  However,
on MS-Windows the backslash is a path separator and "path\[abc]" is still seen
as a wildcard when "[" is in the 'isfname' option.  A simple way to avoid this
is to use "path\[[]abc]", this matches the file "path\[abc]".

This fix simply implements this pattern for TreeDirNode l:pathSpec, and only on Windows - as I haven't seen any documentation showing this bug on other systems.

Before this change, children of directories with square-bracketed names would not be shown (see issues below for examples).

This is related to, but does not close necessarily vim/vim#12629, neovim/neovim#24233

New Version Info

Author's Instructions

[ ] Derive a new MAJOR.MINOR.PATCH version number. Increment the:
- MAJOR version when you make incompatible API changes
- MINOR version when you add functionality in a backwards-compatible manner
- PATCH version when you make backwards-compatible bug fixes
[ ] Update CHANGELOG.md, following the established pattern.
Collaborator's Instructions
[ ] Review CHANGELOG.md, suggesting a different version number if necessary.

[ ] After merging, tag the commit using these (Mac-compatible) bash commands:

git checkout master
git pull
sed -n "$(grep -n -m2 '####' CHANGELOG.md | cut -f1 -d: | sed 'N;s/\n/,/')p" CHANGELOG.md | sed '$d'
git tag -a $(read -p "Tag Name: " tag;echo $tag) -m"$(git show --quiet --pretty=%s)";git push origin --tags

preservim / nerdtree