Convert Directory and File classes to JavaScript

[winstliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Converts the Directory and File-related classes to JavaScript. I plan on investigating switching tree-view over to @atom/watcher, which the JavaScript conversion will help with.

Alternate Designs

None.

Benefits

JavaScript!

Possible Drawbacks

Potential regressions. I'd like to think that spec coverage is pretty good for tree-view but I'll be using this branch for a while before merging to play it extra safe.

Applicable Issues

None.

[daviwil]

I considered doing this too but I wondered if it'd totally break a bunch of pending PRs. Is there anything we absolutely want to merge before doing this?

[winstliu]

@daviwil Thanks for bringing that up. I've identified the following PRs that also modify one of the files changed in this PR:
#1144
#1048
#955 (already conflicting)
#744 (already conflicting)
#658 (already conflicting)
#614 (already conflicting)
#469 (already conflicting)
#344 (already conflicting)

Therefore I believe the ones we should look at and potentially merge before this one are #1144 and #1048. It looks like @lee-dohm is already helping with #1048, which only leaves #1144.

[daviwil]

Just merged #1144 and #1048! Proceed :)