Fix the scope of several key mappings

[PhilRunninger]

Description of Changes

Closes #1149

This pull request changes the scope of some of the key mappings. Prior to this, they were scoped to run on all node types: directory and file. But they didn't make sense to be run on directory nodes, as this creates another NERDTree in the new window. The key mappings whose scopes were changed are: i, s, gi, gs, and go. They now work only on file nodes and file bookmarks.

When testing, it was discovered that the go command on directory bookmarks wasn't working correctly (probably because NERDTreeFind was changed some time back). Therefore, s:previewBookmark() was rewritten so that the bookmark directory would become the root of the NERDTree.

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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 merge, tag the merge commit, e.g. git tag -a 3.1.4 -m "v3.1.4" && git push origin --tags

[lifecrisis]

This all looks good to me. I did look at the documentation, and I don't think anything there needs to be updated. @PhilRunninger, you might ought to check just to be sure I didn't miss anything.

The docs do confirm, for example, that the intended behavior of i is to operate only on files and bookmarks, not directories. Thus, this PR is in line with the contract that our documentation provides.

[lifecrisis]

Looks great, @PhilRunninger! Thanks!