Refactor sort comparison functions, removing redundancy

[PhilRunninger]

Description of Changes

When researching #1162, I discovered a pair of functions that were doing very much the same thing: Path.compareTo(path) and nerdtree#compareNodesBySortKey(n1,n2). I removed the compareTo function, and used the more robust compareNodesBySortKey function instead.

Speaking of compareNodesBySortKey, there was another function named compareNodes that appeared to be unused. On the off chance that was written for other plugins, I decided to keep that function, refactored to fit the new structure.

I also added a new function to accept two path objects instead of node objects. It's named nerdtree#compareNodePaths, and to keep things simple, it is called by nerdtree#compareNodes.

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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